Cook noodles according to package instructions. While noodles are cooking, prepare sauce (directions below).
Drain noodles; stir in Greek yogurt, chicken breast, artichoke hearts, and spinach.
Combine pasta with sauce and season with additional salt, pepper, Parmesan cheese, and fresh lemon juice to taste.
Sauce: Melt butter in a small saucepan.
Stir in onions and garlic and saute 2 minutes.
Stir in flour, salt, and pepper.
Stir in milk.
Cook, stirring continuously, over medium heat until thickened and bubbly.
Stir in Parmesan cheese.