Coat a large high-sided sauté pan over medium-high heat with the olive oil. Add the lentils, garlic cloves and bay leaves with 4 cups water or just enough to cover. Bring the water to a simmer and lower the heat. Simmer the lentils, uncovered, until they are tender, 25 to 30 minutes. (If you need to add water for the lentils to finish cooking, add a little at a time —there will be more flavor in the lentils if you cook them and end up with very little liquid at the end of the process.)
Once the lentils are cooked but still somewhat al dente, set them aside to cool. Remove and discard the garlic cloves and the bay leaves. Season with just a touch of salt and pepper to taste. Retaste, but hold off on reseasoning until serving.
Add the duck fat to a large sauté pan over medium heat. Add the onions, season with salt to taste and allow them to caramelize, about 5 minutes per side.
When you are ready to serve, add the lentils to the sautéed cipollini onions. Stir to coat the lentils with the duck fat. Taste and season with salt, if needed, and stir in the vinegar to taste. Transfer to a serving bowl.
Start the lentils while the duck is roasting, then sauté the onions while it rests.