Preheat oven to 350 degrees.
Bake chicken breasts covered in cream of chicken soup and dried thyme for about 40 minutes or until chicken is cooked through.
Meanwhile, sauté onion in olive oil until tender and translucent; add in garlic and sauté an additional minute. Reserve. Add sliced mushrooms to pan and brown both sides for 7-8 minutes.
Meanwhile, prepare large shell pasta, according to package instructions. Drain.
Remove chicken from sauce and cut into cubes. Combine with thawed spinach, sautéed onion and garlic with cream cheese and a handful of shredded cheeses. Season will salt & pepper.
Fill each shell with chicken mixture. Arrange in baking dish, top with sauce, sautéed mushrooms, and remaining shredded cheese.
Bake for 20 minutes.