Wash the spinach and remove the hard ends of the stems. Heat the butter in a pan and sauté the finely diced onions. Chop the spinach leaves into large pieces and add to the pan with the onions for approx. 1 minute (until the spinach is wilted). Then remove from the pan from the heat and let cool.
Crack 4 Dr. Eggs into a bowl and add the sour cream. Add salt and pepper and mix with a whisk. Add the cooled spinach and onion mixture.
Dice the cheddar into small pieces and put aside. Heat two tablespoons butter in a pan and pour the egg mixture into the pan. Sprinkle the diced cheddar onto the omelette. Cook the omelette slowly over low heat and cover shortly with a lid to thoroughly cook the top part with the cheese.