Cook the pasta as per the instruction on the package, drain and set aside (at this point, you can spray a little cooking oil on the fettuccine pasta to prevent them from sticking together).
Take a large skillet, heat the butter or olive oil (whichever you are using), and add chopped garlic and onions, sauté for 2-3 minutes.
Add in the already cooked feta chicken sausages and cook for 4-5 minutes.
Toss in the rosemary, tomatoes, fettuccine, spinach, and cheeses. Let it cook for 5 minutes.
Lastly, add in the pasta sauce, mix well, simmer for a minute and serve hot!