Chicken Tetrazinni

• 4 chicken breasts (boneless)
• 1 onion (chopped)
• 230 gr cheddar cheese (grated)
• 160 gr low fat cheddar (shredded)
• 240 ml low-fat chicken broth
• 120 ml non-fat sour cream
• 120 ml cream sherry
• 1 package spaghetti noodles (cooked)
• 1 package spec req mushroom soup