In a medium bowl, whisk together the eggs, milk, salt, and black pepper.
Heat olive oil in a skillet over medium heat. Add ground beef and cook until browned, about 5 minutes. Remove beef from skillet and set aside.
In the same skillet, add diced bell peppers and onion. Cook until vegetables are tender, about 5 minutes.
Return the cooked ground beef to the skillet and mix with the vegetables.
Pour the egg mixture into the skillet. Cook for 3-4 minutes, or until the edges start to set.
Sprinkle feta cheese and chopped parsley over the omelette.
Using a spatula, fold one side of the omelette over the filling. Cook for another 2-3 minutes, or until the center is cooked to your liking.
Slide the omelette onto a plate and serve hot.