Broccoli Gratin

Bahan-bahan (10 porsi)

• ¼ cup (½ stick) butter
• ⅓ cup minced onion
• 2 large garlic cloves, (minced)
• ½ teaspoon dry mustard
• 4 tablespoons Dijon mustard, (divided)
• 2 cups fresh breadcrumbs ((see recipe notes below))
• 3 ounces (1 cup packed) Parmesan cheese, (divided)
• 12 cups broccoli florets ((about 3 pounds full crowns))
• ¾ cup whipping cream