1. Melt the butter and olive oil in a nonstick frying pan and fry up the chicken on both sides for about 7 minutes. 2. Season with salt and pepper to taste and stir in the tomato sauce and chopped onions. 3. Cut the buns in half and spread the chicken and tomato sauce all over. 4. Top with Greek yogurt, baby spinach, and grated cheese. 5. In a bowl add the melted butter, chopped parsley, salt and pepper to taste and minced garlic. 6. Mix until combined and brush the buns all over. 7. Let them bake in the oven for about 15 minutes in a preheated oven at 350 F or 180 C degrees. 8. Serve and enjoy.