Preheat the oven to 350f / 180c.
Put the oil in a skillet and place on a medium high heat on the stovetop. Once the oil is hot, add the ground beef.
Break up the beef with a spatula, add the salt and pepper, and cook til browned.
Remove the beef from the skillet with a slotted spoon, leaving the oil and fat in, and place to one side.
Put the skillet back on the heat and add in the chopped vegetables, oregano, salt and pepper. Cook, stirring occasionally til softened. Take off of the heat and set to one side.
In a jug, stir the cream into the marinara sauce. Set to one side.
Peel the potatoes and slice them as finely as you can.
Layer the potato slices evenly in the bottom of the casserole dish. spread the cooked ground beef on top, then the cooked vegetables. Pour over the creamy marinara sauce and sprinkle over the parmesan.
Cover the casserole dish tightly with foil and place in the preheated oven. Cook for 1 hour
After 1 hour of cooking, carefully remove the casserole from the oven and remove the foil.
Place the sliced mozzarella on top and return to the oven for around 10 minutes til the cheese has melted and browned.
Let sit for 10 to 15 minutes before serving.