Brussels Sprout & Leek Gratin with Rosemary

Ingredients

• 2 tablespoons unsalted butter
• 1 large onion, thinly sliced
• 2 leeks, white and light green parts only, thinly sliced
• 1 pound Brussels sprouts, trimmed and halved
• 1 cup grated Gruyère cheese
• 3/4 cup shredded Parmesan cheese
• 1/2 teaspoon salt
• 1/4 teaspoon freshly ground black pepper
• 1 tablespoon chopped fresh rosemary