Heat 2 tablespoons of oil in a skillet. Add cubed chicken and cook over medium-high heat, stirring frequently, until cooked through and no longer pink in the middle, about 8 minutes. Drain excess fat. Set aside.
Heat 1/4 cup olive oil in a saucepan on medium 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 sauce until it starts to bubble (a.k.a. simmer), then turn the heat to low, and simmer uncovered for 25 minutes, adding chicken when there's 5 minutes left to go.
For the fusilli, 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 fusilli is cooked, drain it and put it in a large bowl. Add the sauce. Serve!