Prep Time: 40 minutes
Cook Time: 1 hour 15 minutes
Total Time: About 1 hour 55 minutes (plus optional resting)
Difficulty: Medium – requires careful slicing and a multi-step sauce.
Intensity: Moderate effort, with a high-flavour, impressive result.
Ingredients
For the Vegetables & Layers:
600g sweet potato (orange or white), peeled
600g parsnips, peeled
500g waxy potatoes (e.g., Desiree), peeled
1 large brown onion, finely sliced
2 cloves garlic, minced
2 tsp fresh thyme leaves, plus extra for garnish
2 tbsp olive oil
Salt and black pepper, to taste
For the Bush Spice Blend:
1 tsp ground wattleseed (or substitute with finely ground coffee for a hint of bitterness)
1 tsp ground bush tomato (or substitute with smoked paprika)
½ tsp ground pepperberry (or substitute with regular black pepper and a tiny pinch of cayenne)
1 tsp dried ground lemon myrtle (crucial – do not substitute)
For the Cream Sauce:
60g unsalted butter
60g plain flour
750ml full-cream milk, warmed
150ml pure cream
1 vegetable stock cube, crumbled
1 bay leaf
Freshly grated nutmeg, to taste
For the Cheese Topping:
100g Gruyère cheese, coarsely grated
100g vintage (sharp) cheddar, coarsely grated
Equipment:
Mandoline or very sharp knife for even slicing
Large rectangular baking dish (approx. 25cm x 35cm / 9″x13″)
Medium saucepan
Whisk
Method
Step 1: Prepare the Vegetables & Spice (15 mins)
Preheat & Prep: Preheat your oven to 180°C (160°C fan-forced / 350°F). Lightly grease your baking dish.
Slice Evenly: Using a mandoline or sharp knife, slice all peeled root vegetables into even 3mm rounds. This is key for even cooking. Keep the sweet potato separate as it can discolour.
Create the Bush Spice: In a small bowl, mix the ground wattleseed, bush tomato, pepperberry, and lemon myrtle. Set aside.
Sauté Aromatics: In a large frying pan, heat the olive oil over medium heat. Add the sliced onion and cook for 8-10 minutes until soft and translucent. Add the minced garlic and thyme, cooking for a further 1 minute until fragrant. Remove from heat.
Step 2: Make the Bush-Spiced Cream Sauce (15 mins)
Make a Roux: In your medium saucepan, melt the butter over medium heat. Add the flour and whisk constantly for 2 minutes until it forms a sandy paste (a “roux”) and smells slightly nutty.
Incorporate Milk: Gradually add the warm milk, about 100ml at a time, whisking vigorously after each addition until completely smooth before adding more. This prevents lumps.
Flavour the Sauce: Once all milk is added and the sauce is thickened, stir in the cream, crumbled stock cube, bay leaf, and a generous grating of nutmeg. Add half of your prepared bush spice blend (save the rest for layering). Whisk well.
Simmer: Reduce heat to low and simmer gently for 5 minutes, whisking occasionally, until the sauce coats the back of a spoon. Season with salt and pepper. Remove the bay leaf.
Step 3: Assemble the Gratin (10 mins)
First Layer: Spread half of the sautéed onions evenly over the base of the greased dish. Arrange a single, overlapping layer using half of the potato slices. Season lightly with salt and pepper.
Second Layer: Scatter over half of the remaining bush spice blend. Pour over one-third of the cream sauce.
Third & Fourth Layers: Create a layer with all of the parsnip slices. Pour over another third of the sauce. Follow with a layer using all of the sweet potato slices, seasoning as you go.
Final Vegetable Layer: Sprinkle with the last of the bush spice. Top with the remaining half of the potato slices.
The Final Pour: Pour the remaining cream sauce evenly over the top, gently shaking the dish to let it seep down. It will seem generous but will be absorbed during baking.
Step 4: Bake to Golden Perfection (1 hour 15 mins + resting)
Cover & Bake: Cover the dish tightly with aluminium foil. Bake in the preheated oven for 45 minutes.
Add Cheese & Crisp: Carefully remove the foil. The vegetables should be nearly tender when pierced with a knife. Evenly sprinkle the mixed Gruyère and cheddar cheeses over the top.
Finish Baking: Return the dish, uncovered, to the oven for a further 25-35 minutes, or until the top is deeply golden and bubbling, and the vegetables are completely tender.
Rest: This step is crucial! Let the gratin rest at room temperature for at least 20-25 minutes before serving. This allows the sauce to set slightly, ensuring you serve perfect slices rather than a soupy mixture.
Garnish & Serve: Garnish with extra fresh thyme leaves. Scoop out generous portions with a large spoon or spatula.
Chef’s Notes & Tips for Success
Slice Consistency is Key: A mandoline ensures uniform 3mm slices, guaranteeing everything cooks at the same rate.
Make Ahead: You can assemble the entire gratin (up to the point of adding the cheese) up to 24 hours in advance. Cover and refrigerate. Let it sit at room temperature for 30 minutes, then add cheese and bake as directed, adding 5-10 minutes to the covered baking time.
Botanical Substitutions: If you cannot source Australian bush spices, use the suggested substitutes. However, for the true flavour profile, seek out lemon myrtle – it’s the star.
Leftovers & Reheating: Leftovers are divine. Reheat, covered with foil, in a 160°C oven until warmed through, or in portions in the microwave.
Nutrition Information (Per Serving, based on 8 servings)
Calories: ~485 kcal
Protein: 15g
Total Fat: 28g (Saturated: 16g)
Carbohydrates: 42g (Dietary Fibre: 8g, Sugars: 13g)
Sodium: ~450mg