Melt the butter in a skillet and add the onions, mushrooms, celery and salt. Cook till they become tender and the liquid from the mushrooms has cooked off. Stir in the garlic and cook for 1 minute. Remove from the heat and allow to slightly cool. Pour the mixture into a bowl and add the egg, breadcrumbs, mozzarella, parmesan cheese and basil. Stir to combine well and pour into a lightly greased 1 1/2 quart dish.
Top with the provolone, cutting in half to fit the edges. Sprinkle with a little more basil if desired. Bake in a 350 degree oven for 20 minutes or until the top is golden brown and bubbly.