Preheat the oven to 230ºC .
Melt the butter or margarine in a frying pan. Add onion and celery and cook until softened, about 5 minutes. Remove the pan from the heat. Add the bread cubes, thyme, and season with salt and pepper to taste. Stir to mix well.
Season the cavity of each trout with salt and pepper. Stuff each trout with the bread mixture, dividing it evenly among the fish. If necessary, secure the openings with wooden toothpicks.
Wrap 2 bacon slices around each stuffed trout. Arrange in a baking dish, in one layer. Bake until the fish flakes easily when tested with a fork and the bacon is crisp, 35-40 minutes. Serve garnished with celery leaves or sprigs of parsley.