Dice onions & tomatoes, mince garlic. Set aside.
Measure flour, salt, ground white pepper into one mixing bowl. Set aside.
(if you are grating any of the cheese yourself…) grate cheese as directed. Set aside.
Slice chicken into 1“ or smaller pieces, lightly coat with olive oil.
In a large skillet, cook chicken over medium heat for about 7 minutes.
Start cooking fettuccini pasta with a little salt, usually boiling about 13 minutes.
Add 2 tbl butter to the chicken, half the garlic (2 1/2 cloves), and Italian seasoning. Mix well and cook until chicken is done, then set aside.
In a stock pot, add 4 tbl butter, onion, the rest of the garlic (2 1/2 cloves). Cook until onions are tender and translucent.
Stir in flour, kosher salt, ground white pepper. Cook for a minute or two over medium heat, mixing and stirring often.
Slowly add milk, half & half. Stir until creamy.
Stir in all cheese until melted.
Stir in chicken mixture, roma tomatoes, and sour cream.
Serve over cooked fettuccini, or mix everything together if preferred.