Heat the oil in a large non-stick frying pan and cook the onion for 5-6 minutes until softened. Add the peas and toss for 1 minute to soften. Remove from the heat, add the basil leaves, and toss together to wilt a little. Transfer all the ingredients to a greased 20cmx20cm square baking tin or a 23cm non-stick frying pan. Take the Stilton and cheddar and sprinkle over the pea mixture.