Season the steak with 1 teaspoon herbes de Provence and salt and pepper. In a heavy skillet, melt 2 tablespoons butter over medium-high heat. Add the steak and cook for 5 minutes on each side for medium-rare. Transfer to a plate, reserving the skillet.
Meanwhile, bake the french fries according to package directions.
Return the skillet to medium heat, add 1 tablespoon butter and the onion and cook, stirring frequently, until softened and golden, about 10 minutes; season with salt and pepper. Push the onion to one side of the skillet and melt the remaining 1 tablespoon butter on the other side. Whisk the flour into the butter and cook, whisking, for 2 minutes. Whisk the beef broth into the flour mixture, incorporating the onions, and bring to a boil, whisking frequently, until the gravy has thickened. Cover and remove from the heat.
Thinly slice the steak against the grain. Pour any juices from the plate into the gravy and stir to combine. Arrange 2 slices of toast on each of 4 plates. Mound with french fries and steak slices and smother with the gravy.