Whisk the yogurt, lemon juice, minced garlic, and salt together in a small bowl. Set this aside to allow the flavors to marry while you prepare the main dish.

Place the rice in a medium bowl and cover with hot tap water. Soak for 15 minutes, then drain and rinse under cold water until the water runs clear to prevent the grains from sticking together.

Toss the sliced onions with salt in a large microwave-safe bowl. Microwave until the onions have softened and released significant liquid, about 10 minutes, then rinse under cold water and dry very thoroughly using a salad spinner or clean kitchen towels.

Heat the vegetable oil in a large Dutch oven over medium-high heat until it shimmers. Fry the onions in two batches until they are deeply browned and crispy, about 8 to 10 minutes per batch. Use a slotted spoon to transfer them to a paper towel-lined plate and season immediately with salt.

Combine the lentils with 3 cups of water and salt in a medium saucepan. Bring to a boil, then reduce heat and simmer until the lentils are partially tender but still firm in the center, about 15 minutes. Do not drain the lentils; you will use the cooking liquid for the rice.

Carefully pour the frying oil into a heatproof container, reserving 3 tablespoons in the Dutch oven. Cook the minced garlic, coriander, cumin, cinnamon, allspice, and cayenne over medium heat until the spices are fragrant, about 1 minute.

Stir in the soaked rice and cook until the edges of the grains are translucent and smell nutty, about 2 minutes. Add the lentils along with their cooking liquid, the sugar, and more salt and pepper.

Bring the mixture to a simmer, then cover tightly and reduce the heat to low. Cook until the liquid is fully absorbed and the rice is tender, about 15 minutes. Remove from the heat and let the pot stand, still covered, for 10 minutes to finish steaming.

Fluff the rice gently with a fork and fold in the cilantro. Serve hot, mounding the crispy fried onions generously on top and serving the yogurt sauce on the side.