Preheat the oven to 190C/Gas 5. In a medium saucepan, heat the olive oil. Add the onion and cook over a medium to low heat until transparent. Add the garlic and cook for a further minute. Add the tomatoes, breaking them up with a spoon and cook for 15 minutes over a low to medium heat. Add the basil and oregano, salt and pepper and continue to cook for another 5 minutes then set aside.
Remove the skin from all the sausages and put the sausage meat into a bowl. Roll into small balls and fry in a large frying pan until golden and cooked through. Set aside.
Spoon 1/2 of the tomato sauce into an oven dish and top with a single layer of lasagna sheets. Spread 1/3 of the ricotta on top, then 1/3 of the mozzarella. Scatter over 1/2 of the sausage balls and finally 8 slices of pepperoni. Repeat the entire process again starting with the tomato sauce until you are left with a layer of pasta. Spread the remaining 1/3 of ricotta on top, followed by the remaining 1/3 of mozzarella and the final 8 slices of pepperoni.
Bake for approximately 45 minutes. Remove from oven and leave to sit for 5 minutes. Cut and serve immediately.