Heat the olive oil in a large saucepan over medium high heat. Add the diced onion, carrot and potato. Sautee for about five minutes until onions have softened, stirring occasionally.
Rinse the lentils well, removing any debris. Add to the saucepan, and saute for three minutes, stirring constantly so they don't stick to the bottom of the pan.
Add the six cups of water, the stock cubes, and all the dried herbs plus the salt and pepper.(The dried mint all the way through to black pepper). Let the soup come to a boil, stirring occasionally, then reduce heat to medium and let the mixture cook uncovered for 25-30 minutes, or until lentils and vegetables are cooked through.
In a separate small skillet, add the butter and let it melt over medium heat, then add the flour, and whisk constantly until the mixture bubbles and thickens slightly. Add this to the lentil soup pot.
Once soup is totally cooked through, use an immersion blender or a normal blender (And blend in batches), and blend soup to desired consistency. I like it creamy and smooth but with a few chunks of veggies.
Serve hot, with a slice of lemon, and green olives on the side.