Mornay and Mushroom Duxelles Burger With Crispy Shallots Recipe

Name: Mornay and Mushroom Duxelles Burger With Crispy Shallots Recipe
Ingredients: 3 medium shallots, roughly chopped; 20 ounces cremini mushrooms, roughly chopped; 4 1/2 tablespoons unsalted butter, divided; 1 tablespoon loosely packed fresh marjoram leaves, finely chopped; Kosher salt and freshly ground black pepper; 1 1/2 tablespoons all-purpose flour; 1 cup milk; 3/4 cup grated Gruyere cheese (about 4 ounces); 1 1/2 pounds freshly ground beef chuck, preferably about 80 percent lean, formed into 4 (4 1/2 inch wide) patties; 4 onion rolls, split and toasted; 1 recipe Fried Shallots