There’s a dish I had years ago in Piedmont that I still think about — thin potato slices bathed in a cream sauce built almost entirely from wild porcini, with just enough Parmigiano on top to form a crust worth fighting over. I couldn’t stop thinking about it, and I couldn’t recreate it exactly, because fresh porcini just isn’t something you casually pick up at most grocery stores.
So this is the version that actually gets made in my kitchen. A mix of shiitake, cremini, and plain white button mushrooms gets you most of the way there — not identical to porcini, but earthy and deep in its own right, and infinitely easier to find. The mushrooms get sautéed down, then simmered into milk and cream until the whole thing turns into something rich enough to spoon over the potatoes like a sauce that means business. Add Parmigiano-Reggiano, let it bake until the top goes deep gold and slightly crisp, and you’ve got something substantial enough to eat as a vegetarian main with a sharp, simple salad — or to set proudly next to a Sunday roast.
What You’ll Need
The vegetables:
- 2 1/2 lbs. waxy potatoes (Yukon Golds work well), peeled and sliced 1/8-inch thick
- 8 oz. shiitake mushrooms, cleaned, stems discarded, sliced
- 8 oz. cremini or white mushrooms, cleaned, trimmed, chopped
- 1 garlic clove, minced
The cream sauce and seasoning:
- 2 tablespoons unsalted butter
- 1 cup heavy cream
- 1/2 cup whole milk
- Kosher salt and freshly ground black pepper, to taste
The topping:
- 1/2 cup Parmigiano-Reggiano, freshly grated
How to Put It Together
1. Prep and preheat
Get your oven going first — preheat to 375°F (190°C), and grease a large baking dish while it heats.
2. Sauté the mushrooms
Melt the butter in a large skillet over medium heat, then add the shiitake and cremini mushrooms. Let them cook, stirring fairly often, for about 5 minutes. You’re waiting for the liquid they release to fully cook off — don’t rush this part, it’s what keeps the gratin from turning watery later.
3. Build the cream sauce
Once that liquid’s gone, stir in the garlic along with salt and pepper to taste, and keep cooking for another 5 minutes, until the mushrooms have taken on real color and gone golden brown. Then pour in the heavy cream and milk, bring it to a gentle simmer, and pull the skillet off the heat. You want the sauce warm and just starting to thicken, not fully reduced — it’ll continue cooking in the oven.
4. Layer the gratin
Now the layering, which is really just repetition. Spread a third of the potato slices across the bottom of your baking dish, season lightly with salt, and pour over half the mushroom-cream mixture. Repeat with another third of the potatoes, a little more salt, and the rest of the mushroom sauce. Finish with the last third of potatoes arranged neatly on top, seasoned with both salt and pepper this time.
5. Bake to golden perfection
Scatter the Parmigiano-Reggiano evenly over that top layer and slide the dish into the oven. Bake for 45 to 60 minutes — you’re looking for potatoes that give no resistance at all and a cheese crust that’s bubbling and deeply browned, not just melted.
6. Rest before serving
Let it sit for 5 minutes once it comes out. It’s tempting to skip this, but that short rest is what lets the sauce settle instead of running everywhere the second you cut into it.