Pick over the red lentils and discard any misshapen lentils or stones.

Rinse the lentils and drain.

In a saucepan over medium heat, melt the butter.

Add the chopped onion and saute, stirring occasionally, until tender and translucent (8 to 10 minutes).

Add the coriander, cumin, ginger, turmeric, and cayenne pepper and stir to mix well.

Reduce the heat to low and cook, stirring occasionally, to release the flavors of the spices (2 to 3 minutes).

Add the lentils and then gradually add the vegetable or chicken stock, stirring constantly.

Bring to a boil over high heat, then reduce the heat to low, cover partially, and simmer until the lentils are very soft (30 to 40 minutes).

Remove from the heat and let cool slightly.

Working in batches, puree the soup in a blender or food processor until smooth.

Return the soup to a clean saucepan and place over medium heat.

Stir in the diced tomatoes and lemon juice and cook until heated through.

Season with salt and fresh ground black pepper to taste.

Garnish with fresh mint or cilantro before serving.