Heat the olive oil in a large saucepan. Add the onion and cook on a low heat, stirring often until soft and golden brown. Add the garlic, cook for a minute, then add the diced carrot and pepper. Cover the pan with a lid and sweat on the lowest heat until soft. This should take 10-15 minutes.
If preparing in advance don't add the tortellini until you're ready to serve as it will soak up all the liquid and become overcooked. Simply reheat the soup and add the pasta when you're ready to eat - fresh tortellini will only take 2-3 minutes to cook.