French Onion Green Bean Casserole

Bahan-bahan (8 porsi)

• 4 medium yellow onions (halved lengthwise and thinly sliced)
• 5 tablespoons unsalted butter (divided)
• 2 teaspoons kosher salt (divided)
• 1 teaspoon ground black pepper (divided)
• 2 ½ pounds green beans (trimmed and cut into 1 ½-inch pieces)
• 8 ounces baby bella mushrooms (sliced)
• 2 tablespoons all-purpose flour
• 2 ½ cups 2% milk (plus additional as needed)
• ⅛ teaspoon ground nutmeg
• 1 cup croutons (crushed)
• ¼ cup freshly grated Parmesan cheese
• 1 ½ cups shredded Gruyere cheese (divided)