Whisk the yogurt, lemon juice, minced garlic, and salt together in a small bowl until smooth. Cover and refrigerate until serving time to allow the flavors to meld.

Toss the onions with salt in a large microwave-safe bowl and microwave for 5 minutes until softened. Rinse the onions thoroughly under cold water, drain, and pat them dry with paper towels to ensure they fry properly rather than steam.

Combine the onions and oil in a Dutch oven over high heat. Cook, stirring frequently, until the onions are deeply golden brown and crispy, about 25 to 30 minutes. Drain the onions through a colander set over a bowl, reserving the aromatic oil.

Bring the lentils, 4 cups of water, and salt to a boil in a medium saucepan. Reduce the heat to low and simmer until the lentils are tender but still hold their shape, about 15 to 17 minutes, then drain and set aside.

Place the rice in a bowl and cover with hot tap water by 2 inches; let stand for 15 minutes. Swish the grains gently to release starch, then drain and rinse with cold water 4 to 5 times until the water runs clear, ensuring the final dish is fluffy rather than gummy.

Heat 3 tablespoons of the reserved onion oil in the empty Dutch oven over medium heat. Stir in the garlic, coriander, cumin, cinnamon, allspice, pepper, and cayenne and cook until fragrant, about 2 minutes.

Stir in the drained rice and cook until the edges of the grains begin to turn translucent, about 3 minutes. Add 2¼ cups water, sugar, and salt and bring to a boil.

Stir the cooked lentils into the rice, reduce heat to low, cover, and cook until the liquid is fully absorbed, about 12 minutes.

Remove the pot from the heat. Place a clean, folded dish towel over the pot and secure the lid on top; let it sit for 10 minutes to absorb excess moisture.

Fluff the rice and lentils with a fork and stir in the cilantro and half of the fried onions. Serve on a large platter topped with the remaining crispy onions and the chilled yogurt sauce.