In a cast iron or other ovenproof skillet, sauté the leeks in 1 Tbspn of each butter and olive oil until soft and beginning to brown. Remove with a slotted spoon. Beat eggs with yogurt or milk and seasonings. Stir in the leeks. Melt the remaining butter with the remaining olive oil in the pan and pour in the egg mixture. Cook over medium heat about 5 minutes until underside is golden. Sprinkle cheese on top and place under the broiler for a few minutes until the frittata puffs and browns. Cut into wedges and serve. Makes 4 to 6 servings.