Cook the Lentils: In a medium saucepan, combine the rinsed red lentils, water, oil, and salt.
Bring to a boil over medium-high heat, then reduce the heat to a simmer.
Cook, uncovered, for 20 minutes, stirring occasionally. Skim off any foam that rises to the surface.
Add Potatoes: Stir the diced potatoes into the lentil mixture and continue to simmer.
Sauté the Aromatics: While the soup simmers, melt the butter in a small skillet over medium heat.
Add the diced onion, minced garlic, turmeric, cumin, coriander, and cayenne pepper.
Sauté for 3-4 minutes, until the onion is soft and the spices are fragrant.
Combine and Simmer: Carefully pour the spiced onion mixture into the soup pot.
Stir well to combine.
Continue to cook for another 20 minutes, or until the potatoes are fork-tender. If the soup becomes too thick, add a splash of water to reach your desired consistency.
Serve: Taste the soup and adjust the seasoning with more salt if needed. Ladle into bowls and top each serving with a small pat of butter.
Add a Bright Finish: A squeeze of fresh lemon juice or a dollop of plain yogurt just before serving can cut through the richness and brighten up all the flavors.
Blend for Creaminess: For a smoother, creamier texture, use an immersion blender to partially or fully blend the soup after the potatoes are cooked.
