Best Ever Green Bean Casserole

INGREDIENTS

• 2 tablespoons all-purpose flour
• 1/2 teaspoon freshly ground black pepper
• 1/4 cup all-purpose flour
• 1 teaspoon kosher salt
• 2 tablespoons plus 1 teaspoon kosher salt, divided
• 2 tablespoons unsalted butter
• 2 cloves garlic, minced
• 2 tablespoons panko bread crumbs
• 1 pound fresh green beans, rinsed, trimmed and halved
• 1 cup half-and-half
• 2 medium onions, thinly sliced
• 12 ounces mushrooms, trimmed and cut into 1/2-inch pieces
• 1 cup chicken broth
• Nonstick cooking spray
• 1/4 teaspoon freshly ground nutmeg