A humble peasant dish, this will be best made with real French _lentilles du Puy_. Adapted from Dorie Greenspan's _Around My French Table_ as published by Caroline Russock at Serious Eats. http://bit.ly/d4IBaV
Pick over the lentils, removing any stones. Rinse under cold water and drain well.
Put lentils in a saucepan. Cover with water. Bring to a boil and cook 2 minutes. Pour into strainer and rinse with cold water. Rinse saucepan as well and return lentils to pan.
Push the clove into the onion and place onion in saucepan. Add carrot, celery, garlic, bay leaf and stock. Bring to a boil, then reduce to a simmer and cook 25-30 minutes until almost tender. Skim off any solids that rise up while cooking. Season with salt and pepper and cook an additional 5-10 minutes until fully tender. Add Cognac, if using.
Drain the lentils (save the cooking liquid if you're planning to reheat them later). Discard the garlic, bay leaf and clove. Dice the vegetables and stir back into the lentils along with the shallot, if using.
Serve hot or at room temperature. Store lentils and cooking liquid separately in airtight containers in refrigerator up to 3 days; reheat lentils in cooking liquid in a covered saucepan over low heat.