Spray an 13x9x2 inch glass baking dish with nonstick cooking spray. Arrange bread in a single layer in the dish, then set aside.
In a large skillet over medium heat, add oil and sauté leeks until soft, about 4 minutes. Add garlic and spinach and, while stirring occasionally, cook spinach until wilted, about 3 minutes. Remove from heat. Spoon evenly over bread.
Sprinkle an even coat of goat cheese crumbles and gruyere over top of spinach mix. Sprinkle half the basil over top the cheese.
Whisk eggs, milk, melted butter, dijon mustard, remaining basil and salt and pepper. Pour evenly over cheese. Cover tightly with plastic wrap and refrigerate for 1 hour or overnight.
Remove dish from refrigerator and allow to sit out at room temperature for 15 minutes.
Preheat the oven to 375.
Bake casserole, uncovered, for 20 minutes. Then loosely cover it with aluminum foil and continue baking for 30-35 minutes, or until the sides are browned and the center is puffy.
Remove from oven and allow to cool for 5 minutes then serve.