On medium/high heat, add the butter to the skillet and the sliced onions, sauté until they become translucent, about 2 minutes. Scrape up any brown bits left behind from the chicken searing.
Add the garlic and continue to sauté until fragrant, about one minute.
Add the flour and continue cooking with stirring for about 1 minute.
Slowly add the chicken stock with stirring.
Add the heavy cream and stir in.
Bring up to a simmer then add the chopped chicken back and the chopped sundried tomatoes. If the sauce is too thick add a little chicken stock to thin it down.
Stir in the grated parmesan cheese.
Taste and add more of the leftover seasoning mix or salt and pepper.
Add the spinach and cooked pasta.
Heat through to wilt the spinach and warm the pasta
Garnish with fresh basil, grated parmesan, chili pepper flakes, and cracked back pepper if you like.
Serve and enjoy!