Preheat oven to 350degrees. In a large skillet, heat butter over medium heat. Add onions; cook and stir under tender and golden brown, 8-10 minutes. Cool slightly., Meanwhile, in a large bowl, combine 1 cup cheese, sour cream, salsa and taco seasoning. Stir in cooled onions., Place 2 tablespoons mixture off center on each tortilla. Roll up and place in a well-greased 13x9-in. baking dish, seam side down. Top with sauce; sprinkle with remaining cheese., Bake, uncovered, until heated through and cheese is melted, 20-25 minutes. Sprinkle with cilantro, if desired.