Place the lentils, half the garlic, the bay leaf and the onion in a medium-size saucepan or soup pot. Add 1 quart water. Bring to a boil, reduce the heat and add salt to taste (about 1 teaspoon). Cover and simmer 30 minutes until the lentils are just tender. Remove the onion and bay leaf, and discard. Drain the lentils through a strainer set over a bowl.
In a large bowl, combine the tuna, parsley, chives and drained lentils.
In a small bowl or measuring cup, whisk together the lemon juice, vinegar, mustard, remaining garlic clove and salt and pepper to taste. Whisk in the olive oil and 2 tablespoons of the broth from the lentils. Toss with the tuna and lentils, and serve.