1/3 cup breadcrumbs or gluten-free crumbs (optional topping)
Olive oil spray or 1 tablespoon olive oil for greasing the dish
Instructions
Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with butter or oil.
Prepare the vegetables by washing, peeling as needed, and slicing evenly. Pat dry if they look wet—this helps them brown nicely.
In a small saucepan, melt the butter over medium heat. Add garlic and sauté 30–60 seconds until fragrant. Whisk in the flour to form a light roux, cooking for about 1 minute.
Slowly whisk in the cream (and stock, if using) until the mixture thickens to a smooth, gravy-like consistency. Remove from heat and season with salt, pepper, and nutmeg. Stir in half of the Gruyère and half of the Parmesan.
Spread a thin layer of the creamy sauce on the bottom of the prepared dish. Layer potatoes and other roots, overlapping slightly. Pour sauce over each layer as you go, so every piece is lightly coated.
Finish with a final layer of veg, then pour any remaining sauce over the top. Scatter the remaining Gruyère and Parmesan, and sprinkle with thyme. If using, top with breadcrumbs or gluten-free crumbs.
Cover the dish with foil and bake for 35–40 minutes, until the vegetables are tender and the sauce is bubbling around the edges.
Remove foil and bake 15–20 minutes more, until the top is deeply golden and crisp. The center should be soft but set; a knife should slide in and out with little resistance.
Let the gratin rest for 10 minutes before slicing. Serve warm, with a flourish of fresh thyme if you like.
Notes
A comforting root-vegetable gratin with a silky cheese sauce and melted Gruyère and Parmesan, featuring layers of potatoes, carrots, parsnips, beets, and onions.
