Preheat oven to 400 degrees. Line a baking sheet with parchment paper, and place the tomatoes face down. Roast in the oven for 10-15 minutes, until the skins wrinkle up.
Let them cool off until you can handle them without burning yourself, and then the skins should come off easily, just pull them off with your fingers.
While the tomatoes are roasting, chop the onion and saute in a large saucepan with the olive oil. Add the garlic and carrots and continue to saute until the onion is translucent.
Add the tomatoes and all their juices to the saucepan. Bring to a boil.
Reduce heat to a simmer and cover the pan. Simmer for two hours.
Add salt and pepper, taste and re-season if desired.
If you are making a ragu sauce, once you get close to the two hour mark, cook your beef in a separate pan and drain. Add to the marinara sauce once it is ready.