The feta gives this omelette a nice tang, so you'll want to try to substitute it with another cheese that has a more complex flavor. I like to use goat cheese or Boursin in omelettes as they both lend a creaminess to the filling, and are easy to find kosher versions in most grocery stores. However, if you prefer a mild cheese, then you can also easily use shredded mozzarella or even a mild cheddar if that's what you like.