Preheat the oven to 350°. In a skillet, cook the sausages in the oil over moderately high heat, breaking up the meat, until browned. Season the ricotta with salt and pepper. Spread 1/2 cup of the marinara sauce in a deep-dish pie plate. Top with 1 flatbread, one-third of the sausage and 1/2 cup each of the ricotta and mozzarella. Repeat the layering twice more. Add 1/2 cup of sauce and the last flatbread. Top with the remaining 1 cup of sauce and 1/2 cup of mozzarella.