Combine lentils, water, and salt in a large pot; bring to a boil, reduce heat to medium-low, and cook at a simmer until the lentils are tender and the liquid is absorbed, about 25 minutes.
While the lentils cook, heat olive oil in a pot over medium heat. Cook and stir onion in hot oil until lightly browned, about 5 minutes. Add sausage links to the pot; cook until browned completely, 3 to 5 minutes per side. Stir garlic into the onion mixture; cook and stir until fragrant, 1 to 2 minutes. Add mushrooms, eggplant, and hot pepper to the pot; season with salt and black pepper. Pour diced tomatoes over the vegetable mixture. Place a cover on the pot and cook at a simmer until the sausage is no longer pink the middle, about 10 minutes.
Remove the sausages from the vegetable mixture to a cutting board; cut into chunks and stir into the lentils. Stir vegetable mixture into the lentils. Cover pot and cook until the vegetables are tender and the flavors blend, about 5 minutes.