Quinoa and Broccoli Casserole

Name: Quinoa and Broccoli Casserole
Ingredients: 1 1/2 cups quinoa; 3 cups chicken stock; 3 teaspoons kosher salt, divided; 12 ounces broccoli florets, chopped; 8 ounces button or cremini mushrooms, sliced; 4 tablespoons unsalted butter, divided; 1/2 large onion, diced; 2 tablespoons dry mustard; 1 1/2 teaspoons smoked paprika; 1/2 teaspoon freshly ground black pepper; 1/4 teaspoon cayenne pepper; 1/4 teaspoon ground white pepper; 3 cups half-and-half; 1 large egg; 16 ounces sharp Cheddar, grated; 1/2 cup mayonnaise