Mix all ingredients for the lentils in a pressure cooker or pot. Add 1 liters (about 1 quarts) of water, and bring to a simmer.
Pressure cooker method.
Cover, and cook on pressure over medium heat for 20 minutes, or until the lentils are tender. If the lentils are still submerged in sauce, reduce the liquids so that the sauce is ample, but not watery. Adjust the seasoning if desired, and serve with hot sauce, if desired.
Cover, and simmer the lentils over medium heat about 1 1/2 hours, or until the lentils are tender and the sauce is not watery. (If the liquids reduce too much during cooking, add a little water to prevent the lentils from burning.)
Sprinkle flour on top of lentil mixture, simmering for about 1 minute just to thicken. Pour lentils into casserole dish.
For the topping, place potatoes in a large saucepan and cover with 2 inches of water. Bring to a simmer over medium-high heat. Reduce heat to medium, partially cover and cook until tender, 10 to 15 minutes. Drain and return the potatoes to the pot. Add buttermilk, butter and 1/4 teaspoon each salt and pepper. Mash with a potato masher until mostly smooth.
Top the lentils with the mashed potatoes. Broil, rotating halfway through, until the potato is lightly browned in spots, 6 to 10 minutes.