1. Bring the water to a boil. Add the lentils, garlic and bay leaf to the boiling water and simmer, partially covered, 15 minutes. Stir in 1 teaspoon of the salt, cover and cook until lentils are tender, about 10 minutes. Drain and transfer to a large bowl; discard garlic and bay leaf.
2. Meanwhile, in a medium bowl, toss 2 tablespoons of the diced onion with 2 teaspoons each of the vinegar and oil. Season with a pinch of salt and set aside.
3. Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add remaining onion and the carrot and celery to skillet. Saute the vegetables until golden, 6 to 8 minutes. Add vegetables to lentils; toss with 3 tablespoons vinegar, 2 teaspoons oil and the mustard; season with remaining salt and the black pepper to taste.
4. In skillet, heat remaining oil over medium-high heat. Add the sausages and cook, turning, until browned, about 5 minutes. Transfer to a cutting board and slice on the diagonal.
5. Add the watercress and apple to the dressed red onion; toss well. To serve, make a bed of lentils on each plate. Top with watercress-apple salad and sausage.