1. Place the lentils, the whole small garlic clove, the bay leaf, and the onion in a saucepan with 1 liter of water.
Bring to a boil, reduce the heat, and add 1 teaspoon of salt.
2. Cover and simmer for 25-30 minutes, until the lentils are tender. If you prefer, soak them in plenty of water for 1 hour or more. Drain them, add fresh water, and boil them for less time; they will be ready in 10 minutes.
3. Remove the garlic, onion, and bay leaf and discard them.
Drain the lentils in a colander over a bowl and reserve 2 tablespoons of their cooking liquid.
4. In a large bowl, mix the tuna, parsley, chives, and drained lentils.
In a small bowl, whisk together the lemon juice, vinegar, mustard, minced garlic, salt, and pepper. Add the olive oil and the 2 tablespoons of lentil cooking liquid and whisk until combined.
5. Pour the dressing over the tuna and lentils and toss gently.
Serve the salad chilled or at room temperature. It is ideal as a main course or a side dish.
