Mix lemon pepper with flour. Dredge the chicken thighs in this seasoning mixture.
Heat 2 Tbsp olive oil on medium-high heat in a large skillet. Add chicken thighs, skin side down, and cook for about 4 minutes uncovered until browned. Don’t overcrowd the pan. Cook in batches if needed. Flip the chicken thighs over and cook for 2 minutes on the other side. Remove chicken from pan and drain excess fat from the pan.
Melt 1 Tbsp of butter in the pan. Add chopped onion and cook for 3 minutes. Add minced garlic and cook, constantly stirring, for about 30 seconds. Add chicken stock and 2 tsp lemon pepper seasoning to the skillet and bring to a boil, scraping up any dark bits on the bottom of the pan (the fond). Add the chicken back to the pan, cover, and let simmer for 40 minutes on low heat.
Bring a large pot of water to a boil and cook spinach fettuccine 2 minutes less than package directions. Drain and keep warm.
When tender, remove the chicken one last time (tent with foil to keep warm). Raise heat to medium. In a small bowl, mash together the softened butter and flour then add this mixture to the sauce, whisking briskly until smooth. Add heavy cream and bring to a boil, then add shredded Asiago and stir until melted. Reduce heat to simmer and keep stirring until the sauce is smooth. Remove from heat. Add 2 Tbsp of freshly-squeezed lemon juice and stir in. Top with chopped green onions.
Add chicken back to the skillet on top of the sauce and green onions. Simmer 3 minutes to rewarm.
Serve over spinach fettuccine, top with chopped parsley and decorate with a few slices of lemon.
Lemons are used in cooking for their tangy flavor and bright aroma. They also help to bring out the flavor in the dish!