Season chicken breast with 1/4 teaspoon salt (optional) and pepper. Heat olive oil in a large skillet over medium-high heat. Saute' chicken until cooked through, approximately 7 - 10 minutes. Remove chicken from skillet and set aside.
Add onion and garlic to skillet and saute' until fragrant, about 1 to 2 minutes. Add tomatoes and cook until they begin to soften, approximately 2 to 3 minutes.
Add chicken broth, juice from a lemon, and the remaining salt (optional), reduce the skillet heat and cook until liquid is reduced by half, approximately 15 minutes.
Add chicken and spinach and cook until spinach is wilted, approximately 2 minutes.
Serve the contents of the skillet on top of the spaghetti squash.