Peel and slice potatoes into 1/8-inch thick rounds—I recommend using a mandoline for consistent slices that cook evenly. Trim the tough ends from asparagus and cut into 2-inch pieces. Mince the garlic cloves finely. Grate both the Gruyère and Parmesan cheeses (or use pre-grated Parmesan as noted). While prepping, preheat your oven to 200°C (400°F) and butter a 9x13 inch baking dish or similar-sized gratin dish generously to prevent sticking.

Combine cream, milk, minced garlic, dried thyme, salt, pepper, and a pinch of nutmeg in a saucepan over medium heat. Warm gently, stirring occasionally, until steam rises and the flavors meld together—about 5-7 minutes. Don't let it boil. I find that whole milk creates a richer, more luxurious sauce, so don't skip it even if you have cream on hand.

Begin layering in your buttered dish with a thin layer of sliced potatoes on the bottom, slightly overlapping for even cooking. Sprinkle with a pinch of salt and pepper. Add a thin layer of asparagus pieces, then scatter a handful of mixed Gruyère and Parmesan over top. Repeat these layers—potatoes, asparagus, cheese—until all vegetables are used, finishing with a final cheese layer (reserve about 1/4 cup total cheese for the top crust). The layering ensures every bite has creamy sauce absorbed throughout.

Pour the warm cream and milk mixture from Step 2 slowly and evenly over the layered vegetables, allowing it to seep down between the layers. Use all of it. Dot the top with small pieces of the remaining butter (about 0.75 tbsp cut into small pats). The warm cream will start cooking the potatoes gently from above, and the butter will melt into the top layer for richness.

Cover the baking dish tightly with aluminum foil to trap steam, which gently cooks the potatoes through without browning the cheese too quickly. Bake at 200°C (400°F) for 30 minutes. The foil acts as a lid, allowing the starches in the potatoes to soften evenly while the cream sauce infuses every layer.

Remove the foil carefully and increase the oven temperature to 220°C (425°F) if you'd like a faster, crispier crust. Bake uncovered for 20-25 minutes until the cheese on top is golden brown and the potatoes are completely tender when pierced with a fork. Watch closely toward the end to avoid burning—oven temperatures vary. For a more dramatic golden crust, I like to use Kraft Parmesan specifically because it browns beautifully and gives a nice salty bite.

Remove the gratin from the oven and let it rest for 5 minutes before serving. This allows the cream sauce to set slightly so it won't be watery on the plate, and it gives you a moment to prepare any side dishes. Scoop portions carefully to keep the beautiful layers intact, and serve while warm.