Broccoli Casserole

Name: Broccoli Casserole
Ingredients: Kosher salt, to taste; 4 heads broccoli, cut into small florets; 2 tbsp. olive oil; 8 tbsp. unsalted butter; 8 oz. white button mushrooms, thinly sliced; 1½ cups panko bread crumbs; 2 tsp. dried sage; Freshly ground black pepper, to taste; 3 cloves garlic, finely chopped; 1 small yellow onion, finely chopped; 3 tbsp. flour; 2 cups milk; 2 tsp. dry mustard powder; ¼ tsp. freshly grated nutmeg; 3 oz. cream cheese, softened; 2 cups shredded cheddar cheese; ½ cup mayonnaise; 2 eggs, lightly beaten