In a medium skillet, heat 1 tablespoon of the butter and the oil. Add the leeks, onion and grlic and saute until soft. Add the potatoes and leftover vegetables and cook until potatoes are lightly browned. Remove from skillet and place potato mixture into a medium bowl. Add cheese and cilantro to bowl, toss and set aside.
In another small bowl, combine eggs, cream, salt, black pepper and cayenne pepper and whisk until blended. Pour over potato/leek mixture.
In the empty skillet, melt the remaining 2 tablespoonsof butter. Pour in the potato/egg mixture and cook slowly over low heat, piercing the top with a fork and gently lifting the bottom. Continue cooking until the bottom is brown and set.
Slide omelet onto a dinner plate, invert the plate with the omelet back into the skillet and cook the remaining omelet side until brown. Serve with salsa.