Wash the lentils and beans, then soak in plenty of water for at least 8 hours, or overnight at room temperature. If your kitchen is warm, then place in the refrigerator.
Drain, and in a heavy-bottomed saucepan add the lentils, beans, and 6 cups of water.
Turn the heat to high and bring to a boil, cover, and lower the heat to simmer for an hour. Skim any foam that forms and stir several times. Cook for another hour, or until soft enough to mash easily.
Drain, and mash lentils with the back of a spoon or use a potato masher. There should still be some whole lentils, but you want a thick, coarse consistency.
In a frying pan, or wok, add 1 tablespoon of ghee or oil over medium heat. Add the onions and cook for 5 to 8 minutes, until softened and translucent. Stirring frequently to keep them from sticking to the bottom of the pan.
Add the ginger garlic paste, cumin, red chili, and sauté for 2 to 3 minutes, until the mixture no longer smells raw. Add the tomato purée and cook just until thickened and slightly darker; about 5 minutes. If you are using crushed tomatoes or fresh diced tomatoes, cook an additional 3 minutes, or until they have broken down and the masala is thick.
Add the mashed dal and 2 cups of water to the tomato mixture, turn up the heat to medium-high and bring dal to a boil, then turn down to low and cover. Simmer for 15 to 20 minutes, stirring occasionally, and adding water if needed to keep the dal from sticking. If you have the luxury of time, simmer longer. Add warm water, if needed, to bring to a thick, but soupy consistency.
Add the garam masala, butter, cream or yogurt, lemon juice, sugar and salt. Simmer for another minute and season to taste, adding salt, ground chili, or an additional squeeze of lemon if needed.
Garnish with an optional drizzle of cream, or yogurt, and a sprinkling of coriander leaves.
Black lentil dal tastes even better the next day, and freezes well, so I recommend that you double, or even triple the amount you cook.
Plan ahead because the black lentils (urad dal) and red kidney beans (rajma) need to be soaked overnight.
Adjust the cream and butter to your taste.
If you have the luxury of time, allow the dal to simmer for longer than suggested. The longer the better within reason.
This dal is typically served with a thick consistency, but you can add water to thin it to your taste.
While not traditional, I added slightly more than a tablespoon of lemon juice, because it tasted better that way to me.