Place 1 cup of water next to the stovetop. Heat the oil with the cumin in a large soup pot over medium heat until cumin is browned, about 2 minutes. Add chili flakes and ground peppercorns, and cook for 30 seconds. Add the onion and salt, and cook until onion is deep browned and crispy, 15-20 minutes, adjusting the heat to medium-low as needed to prevent from burning. Stir often and splash with water, scraping browned bits from the bottom of the pot when the onions begin to stick.

Add 2 cups of water along with the lentils and the turmeric to the pot and cook for 5 minutes, stirring occasionally. Add 10 more cups of water and bring to a boil. Reduce the heat to a gentle simmer, cover and cook for 20 minutes, stirring halfway through.

Stir in the couscous and return the soup to a boil. Cook, partially covered, until the couscous is al dente. Cover the pot and turn off the heat. Let the lentil soup stand for 30 minute to 2 hours to thicken.

Before serving, bring to a boil, reduce the heat to medium-low, and simmer for 10 minutes. Taste for seasoning and serve sprinkled with chopped cilantro and a squeeze of lemon juice.