Lettuce Burger With Feta Cheese & Pesto

Ingredients: 3 oz. ground beef 2 tbsp. red onion 1 tsp. basil pesto ½ tsp. garlic 2 slices tomato ¼ tsp. ground cumin 1 tbsp. feta cheese ½ tbsp. butter 1 tbsp. sour cream 1 large lettuce leaf