Heat the olive oil in a saucepan on medium-high heat.
When the oil is hot, add the onions and garlic. Cook until the onions start to get soft (but before they get brown), about 5 minutes.
Add the tomatoes, tomato paste, salt, pepper, and Italian seasoning. Mash up the whole tomatoes into smaller chunks. Keep heating it until it starts to bubble (a.k.a. simmer), then turn the heat to low, and simmer uncovered for 25 minutes.
For the fettuccini, a good time to start boiling the water is just before you add the tomatoes to the sauce. Fill a large pot with water. Bring to a boil, and cook for the time directed on the pasta box.
When the fettuccine is cooked, drain it and put it in a large bowl. Add the sauce. Serve!