Cook fettuccine according to package directions.
Meanwhile, in a large skillet, heat 1 tablespoon of oil (olive or vegetable). Once hot, salt and pepper your chicken and add it to the oil. Cook just until outside of chicken has all turned white. Remove chicken from pan.
While chicken is cooking, slice onion and pepper and mince garlic (or press through garlic press).
Once chicken has been removed, heat another tablespoon of oil and saute onions and peppers until onions are translucent. (You can also throw some fresh sliced button mushrooms in here too if you like.) Once onion is translucent, add garlic and cook 3 to 4 minutes until garlic is fragrant. Be careful not to burn the garlic, it will get really bitter.
Add chicken back to pan with onions, peppers, and garlic. Cover and cook chicken until done.
While that is finishing up, combine butter and whipping cream in a medium sauce pan or pot. Cook over medium heat until butter is melted. Once melted, add parmesan cheese and whisk until combined. Stir frequently to not scorch the cream. Once the mixture has reached a gentle boil, add 1 tsp corn starch and whisk briskly. (You can create a slurry here by combining 3 tablespoons of the hot butter/cream with the cornstarch in another small bowl to help keep the corn starch from clumping and then add it back, but I usually don’t have a problem with it clumping and that’s just one more step that I’d rather not take… K.I.S.S, right?) The sauce will thicken and when it does, turn off the heat. I usually add a little salt and pepper to my sauce too. Just do it to your taste.
Now you’re done! We usually keep the three components (chicken/veggie mixture, pasta, and sauce) separate because it reheats better that way, but you can combine it all in a big ol’ bowl. Sprinkle some more cheese on top and enjoy!