First, prepare your Brussels sprouts. Trim any brown leaves and cut them in half lengthwise. This helps them cook evenly and allows for maximum flavor absorption. The way the sprouts look after trimming will make you want to eat them right away!
Next, preheat your oven to 400 degrees Fahrenheit and move the rack to the middle position. Preheating is crucial as it ensures that the sprouts will roast properly.
In an oven-proof skillet, add the olive oil and butter over medium-high heat. Allow the butter to melt and foam, which indicates it’s ready for the next step.
Add the trimmed Brussels sprouts to the hot skillet. Sauté them for about 8 minutes, stirring occasionally. You want them to start browning on the outside, which will deepen their flavor. The smell will be irresistible!
Once the sprouts have sautéed to a beautiful golden color, remove the skillet from heat. Stir in the garlic, heavy cream, and half of the Gruyère cheese. The mixture will be creamy and comforting, and the garlic will infuse its flavor throughout.
Top the mixture with the remaining Gruyère cheese and the Parmesan cheese. This creates a cheesy crust that will be the highlight of your dish.
Carefully place the skillet in the preheated oven. Roast the gratin for 15 to 20 minutes. I recommend checking at the 15-minute mark; the sprouts should be tender-crisp, and the cheese should be beautifully melted and slightly browned.
If you desire an extra crispy top, you can turn on the broiler for a couple of minutes, but keep a close eye on it to avoid burning. The bubbling cheese will tempt you to dig in!
Season your Brussels sprouts with salt and pepper to taste. This final touch enhances all the flavors and makes the dish truly unforgettable.
Serve Brussels Sprouts Gratin immediately. The longer you leave it, the more the cheese will set, so enjoy it hot and fresh from the oven!