In a medium nonstick frying pan, melt 2 teaspoons butter over a medium heat. Add onion, gently fry until softened, about 5 minutes.
Remove pan from heat. Stir in rice, 1/4 cup stock, cheeses and thyme. Mix well.
Using a sharp knife, cut a deep horizontal pocket in each chop. Spoon rice mixture into pockets, dividing evenly. Secure openings with toothpicks.
Place the same frying pan over a medium-high heat. Add remaining butter and the chops. Cook until browned, turning once, about 2 minutes per side. Add remaining stock. Reduce heat to medium, cover and cook until chops are cooked through, about 10 minutes. Place chops on dinner plates, remove toothpicks. Garnish with thyme sprigs and serve with pan juices spooned over.