Rinse lentils under cold running water and drain. Place the water and lentils in a saucepan over medium heat and bring to a boil. Cover, reduce heat to medium-low, and simmer until lentils just begin to soften, about 20 minutes. Stir the tomatoes and onions into the lentils; bring to a gentle simmer. Crumble the bouillon cube into the lentil mixture and stir until it’s dissolved. Simmer for about 5 minutes more, stirring frequently. Season with salt and pepper to taste.