In a large skillet over medium-high, add in olive oil, chicken thighs, and onion and cook for 5 minutes, cooking chicken on each side to brown. Season thighs and onions with 2 teaspoon kosher salt and 1 teaspoon black pepper.
After five minutes, add in garlic, italian seasoning, and red pepper flakes and cook for one minute til garlic is fragrant.
Turn heat to high and pour in milk and chicken stock. Add pasta and cook until done (egg fettuccini was done in about 9 minutes) be careful about over flow (a wooden spoon over the top of the pot helps).
Once pasta is cooked and the liquid has significantly reduced, add in tomatoes and spinach. Mix in and cook until spinach begins to wilt.
Add in parmesan cheese and stir in until melted and sauce is thickened.
Serve hot.