Halve and slice the 4 onions into thin, even slices (don't chop them roughly). Heat about 2-3 tablespoons of oil in a large pot over medium-high heat. Fry the onions for 8-10 minutes, stirring occasionally, until they turn brown to dark brown. Add the lentils and 1 liter of water. Cover and cook over medium heat for about 1 hour, until the lentils are tender. If they're still firm, cook for another 10-15 minutes.
Season with salt and pepper to taste. Add the bulgur and stir well. Reduce heat to low, cover, and simmer for 10-15 minutes until the bulgur has absorbed the liquid and the mixture has a thick, cohesive texture.