Melt butter in a large skillet. Add onion and cook over medium heat for 10-15 minutes or until very soft, stirring frequently. Lay tortillas on a flat surface. Place equal amounts of cheeses, pears, carmelized onions and thyme on half of each tortilla; fold over to enclose filling. Place on a grill over low heat and cook for about 3-5 minutes per side or until cheese is melted and tortilla is crisp and golden.