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 skillet, add onion, bacon, salt, and pepper. Cook until bacon is crispy and onion is soft. Drain fat. Pour in tomato soup and stir; bring to boil. Reduce heat to low; add 1 cup Cheddar cheese and the milk. Stir until cheese is melted.
Set oven rack about 6 inches from the heat source and preheat the oven's broiler.
Combine sauce with pasta in a baking dish. Sprinkle with remaining 1 cup cheese. Place under the oven broiler until sauce is thick and cheese browned, 3 to 5 minutes.