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Rinse the lentils well.
Add lentils, water, and onion to a pot. Bring to a boil, then lower the heat and simmer about 15–20 minutes until the lentils are very soft.
In a small pan, melt the butter. Add flour and cook 1–2 minutes while stirring. Do not let it brown too much.
Slowly add the beef broth into the flour mixture while whisking so it stays smooth.
Pour this mixture into the soup pot.
Blend until creamy using an immersion blender or regular blender.
Add salt and black pepper to taste. Simmer another 3–5 minutes if needed.
Serve warm with lemon if you like. Bon appétit.