How to Make It
In a large cast-iron skillet, heat the oil. Add the sausages and cook over moderate heat, turning, until browned and cooked through, about 8 minutes. Let cool slightly. Halve the sausages lengthwise and cook cut side down over moderate heat until browned, about 2 minutes. Transfer to a plate.
Season the tomatoes with salt and pepper, add them to the skillet cut side down and cook over high heat until browned, about 3 minutes. Transfer the tomatoes to a plate.
Melt 1 tablespoon of the butter in each of 2 skillets. Fry 4 eggs in each skillet over moderate heat until slightly runny in the center, about 4 minutes. Season with salt and pepper. Set 2 eggs on each of 4 toast slices. Top with the cheese and sausages, sprinkle with the parsley and close the sandwiches. Serve with the tomatoes.