Cook potatoes in boiling salted water for 20 to 25 minutes or until tender; drain and slice.
Meanwhile, clean morel mushrooms according to directions in note that follows. Drain thoroughly; pat dry with paper towels. Slice mushrooms.
In a large skillet, cook and stir mushrooms, onion, and garlic in the 2 tablespoons butter for 10 minutes or until liquid is almost evaporated.
In buttered 8x8x2-inch baking dish, layer half of the potatoes and half of the mushroom mixture. Repeat layers. Using a fork or wire whisk, combine egg, milk, flour, salt, and pepper; pour over vegetables in the baking dish.
Combine bread crumbs and melted butter. Sprinkle over casserole.
Bake in a 350 degree F oven for 30 minutes or until set. Makes 6 side-dish servings.
To clean morel mushrooms, place mushrooms in a pan or bowl. Cover with water; add a dash of salt. Soak for 10 to 15 minutes. Drain, rinse and repeat two more times.