Melt butter in a large frying pan. Saute diced onion for 2-3 minutes until it softens. Add frozen corn and heat 6-7 minutes, until warm. Chop half of the package of feta into small crumbles, it should be about 1 cup. Add feta, cilantro, cayenne pepper and the juice of two limes, to the corn and stir. Add pepper to taste. Serve warm.