Peel the kohlrabi bulbs using a vegetable peeler or paring knife. Trim off any tough ends.

Slice the kohlrabi as thinly as possible—about 1/8-inch thick. A mandoline slicer works great here for even slices.

Thinly slice the onion if you're using it, and mince the garlic.

Pro Tip: Uniform slices help the gratin cook evenly and look more polished when served.

In a medium saucepan, combine the heavy cream, milk, minced garlic, salt, pepper, and nutmeg.

Warm the mixture over medium heat, stirring occasionally until it’s just starting to simmer (don’t let it boil). Remove from heat.

Pro Tip: Warming the cream mixture ahead helps it absorb better into the kohlrabi and shortens overall baking time.

Lightly grease your baking dish with butter.

Add a layer of sliced kohlrabi at the bottom of the dish, overlapping slightly like you would with scalloped potatoes.

Sprinkle a few slices of onion over the kohlrabi layer.

Pour a bit of the warm cream mixture over the top—just enough to moisten the layer.

Sprinkle a little Gruyère and Parmesan cheese.

Repeat the layering process until all the kohlrabi is used up, finishing with cheese on top.

If using breadcrumbs, sprinkle them over the final cheese layer for extra crunch. Dot the top with small bits of butter.

Pro Tip: Gently press down on the layers as you go to make sure the sauce and cheese are evenly distributed and there are no big air pockets.

Preheat your oven to 375°F (190°C).

Cover the baking dish loosely with foil and bake for 30 minutes.

After 30 minutes, remove the foil and continue baking for another 20–25 minutes, or until the top is golden and bubbly, and a knife slides easily through the kohlrabi.

Pro Tip: If the top isn’t browning enough, switch the oven to broil for the last 2–3 minutes—but keep a close eye to prevent burning.

Let the gratin cool for about 10–15 minutes before serving. This helps the creamy layers set and makes it easier to slice.

Pro Tip: Resting the gratin slightly enhances the flavor and texture—plus, no burnt tongues!