Melt the butter in a large pot over medium-high heat.
Add the onion, carrots, and celery and saute until translucent. Add in the garlic and saute for an additional minute or two, until fragrant.
Add in the salt, curry powder, and garam masala and saute an additional few minutes, stirring often to prevent burning and to distribute the seasoning over all of the vegetables.
Add the chicken broth, lentils, and bay leaf. Stir and reduce the heat to medium.
Simmer for 40-60 minutes, until much of the liquid is evaporated and the lentils are soft and tender. Remove the bay leaf and stir in the lemon juice.
Spoon the lentils over hot cooked rice and garnish with a dollop of sour cream or yogurt.