In a spice grinder, grind the mustard and cumin seeds into a powder.
1 teaspoon black mustard seeds, 1 teaspoon cumin seeds
In the same pot that the onions were fried in, add the spice powder and garlic if using. Turn on the stove to medium. Quickly cook the spice for about 30 to 60 seconds until fragrant.
2 cloves garlic (optional)
Drain the soaked basmati rice and discard the liquid. Add the rice to the pot with the spices. Season with the salt. Add in the cooked lentils and half of the fried onions. Mix. Add the liquid. Stir. Bring to a boil. Cover. Lower heat and simmer 30 minutes. Turn off the stove. Keep the pot covered and rest 10 minutes.
1 cup basmati brown rice, 3/4 teaspoon kosher salt, Reserved lentil boiling liquid plus water to make 2 cups liquid, 1 cup black lentils
Remove the cover. Fluff with a fork. Plate and serve, topped with the fried onions. Enjoy.