Bring a large pot of lightly salted water to a boil. Add rotini pasta and cook for 8 to 10 minutes or until al dente; drain.
Heat oil in a large saucepan or frypan, add onion, bacon, salt and pepper. Cook until bacon is crispy and onion soft. Pour in tomato soup and stir; bring to boil. Stir in Cheddar cheese until melted and thick.
Combine sauce with pasta in a baking dish, trickle milk down the sides of the dish and stir slightly. Sprinkle with remaining cheese. Place under the grill until sauce is thick and cheese browned.