In a pot, heat the oil. Cook the onion, the garlic and the bell pepper until the onion is brown.
Add spinach, the lemon juice and water. Bring it to boil. Wait until the spinach is completely liquid.
Add tomato paste and stir until the sauce is thick
Add the diced tomato and feta cheese. Wait for 5 minutes and stir many times.
Add salt and pepper to taste. Serve on pasta