Mushroom, Thyme & Gruyere Cheese Breakfast Casserole*

1 day old baguette, cubed (about 8 cups) 1 tablespoon butter, plus more to grease the pan 8 oz mushrooms, cleaned and sliced half yellow onion, diced small 1 1/2 cups whole milk 8 whole eggs 1 tablespoon fresh thyme leaves 1/2 teaspoon dry mustard 1/4 teaspoon ground nutmeg salt pepper 1 cup fresh grated…