Sort through the lentils to remove any small stones or debris. Rinse and drain them, then place them in a pot. Add the 3 cups of water and bring to a boil. Cover, reduce the heat to very low, and simmer for about 20 minutes, or until the lentils are tender.
Drain the lentils through a colander, reserving the cooking liquid for later.
Caramelize the onions: In a wide skillet, heat the oil over medium-high heat. Add the 3 large sliced red onions and cook for about 10 minutes, stirring constantly, until they begin to soften and wilt. Reduce the heat to low and continue cooking, stirring frequently and scraping the bottom of the skillet to prevent sticking, until the onions are deeply golden brown and crispy. Watch them carefully to prevent burning. Using a slotted spoon or tongs, remove the onions from the oil and set aside. You’ll use them to garnish the mujadara before serving.
Transfer the oil from the skillet to a pot. Add the small chopped onion and sauté until lightly golden.
Stir in the cooked lentils, bulgur, 1½ cups of the reserved lentil cooking liquid, salt, and ground cumin, if using. Bring to a boil, then reduce the heat to low, cover, and simmer for about 25 minutes, or until the bulgur is tender and the flavors have melded together.
Transfer the mujadara to a serving dish and top with the crispy caramelized onions. Serve with Lebanese fattoush or Middle Eastern salad, and enjoy!