Heat the oil in a large saucepan over medium-high heat. Add the mustard seeds, cover the pan, and cook until the seeds have stopped popping (not unlike popcorn), about 30 seconds. Add the lentils and stir-fry until they turn golden brown, 15 to 20 seconds. Immediately add the onion and garlic, and stir-fry until the onion slices are light brown around the edges, 3 to 5 minu