1. Preheat the oven to 425°. Wash the trout, inside and out, under cold running water and pat them dry with paper towels. Season the trout lightly with salt and pepper, then roll them in flour and brush or shake off the excess. In a heavy 10- to 12-inch skillet, melt 2 tablespoons of butter with the oil over high heat.
2. When the foam subsides, add the trout and cook them for 4 to 5 minutes on each side, or until they are golden brown. Carefully transfer the trout to a plate. In a stainless-steel or enameled skillet, melt another 2 tablespoons of butter over moderate heat.
3. Add the sliced mushrooms, sprinkle them with lemon juice, and shaking the skillet almost constantly, cook for about 3 minutes, or until they glisten with butter and are slightly softened.
4. With a slotted spoon, remove the mushrooms from the skillet and spread them over the bottom of a buttered ovenproof baking dish just large enough to hold the 4 trout in one layer. Melt 1 tablespoon of butter in the skillet, add the scallions and cook them for 1 minute.
5. Then with a slotted spoon transfer them to a bowl. In the remaining tablespoon of butter, and in the same skillet, lightly brown the bread crumbs.
6. Arrange the browned trout (adding any juices that have accumulated on the platter) on top of the mushrooms in the baking dish. Sprinkle them with the crisped bread crumbs and spread the scallions on top.
7. Bake on the middle shelf of the oven for 10 minutes, or until the crumbs and scallions are brown. Serve directly from the baking dish.
