Heat 1 tablespoon neutral oil in a large pot over medium-high heat. Add the onions and sauté for 5–6 minutes or until they begin to soften. Next, add the carrots and potatoes and sauté for 5–6 minutes.
Create a well in the middle of the vegetables and add the remaining 1 teaspoon of oil. Once the oil is hot, add the garam masala and cayenne powder, if used, and let it sizzle in the hot oil for 30–45 seconds.
Toss coat the vegetables in the bloomed spice mixture and season liberally with salt and pepper.
Simmer the yellow lentils:
Add the yellow lentils and then pour in 6 cups of water or vegetable stock. Bring it to a boil and then reduce heat to low. Cover the stew and simmer for 20–30 minutes or until the lentils are tender. Add the remaining 2 cups of water or stock if the liquid absorbs too quickly while simmering.
Sizzle the spices (optional):
Melt the vegan butter into a small saucepan. Once bubbly, add the fenugreek, cumin, and black mustard seeds and cook, constantly stirring, for 1 minute. Once the mustard seeds begin to pop, turn off the heat and immediately pour it into the lentil stew. You'll hear lots of beautiful sizzling noises!
To serve:
Divide the stewed lentils between bowls and serve with cooked basmati rice or naan and cilantro leaves.