Lentils and Caramelized Onion Croquettes

In a medium skillet over low heat, cook the onion in olive oil, covered for 20 minutes. Remove the lid and cook, stirring occasionally, until they're browned, meltingly soft, and full of aroma, about 15 minutes. Season well with salt and pepper. Meanwhile, combine the lentils, celery, carrot, and 2 tsp salt in a saucepan. Add water to cover by 3 inches, bring to a boil, then reduce the heat and simmer until the lentils are soft, about 30 minutes. Drain, reserving the liquid for soup stock. Puree the lentils in a food processor until smooth but still retaining a little texture. Add some of the reserved broth if needed. Mix the lentils with the onion and half the bread crumbs. Season well with salt and pepper, then stir in the egg. Spread the mixture out on a platter or a tray to cool so that it will be easier to handle. Form the mixture into 3-inch ovals or rounds 2 1/2 inches across. Spread the remaining bread crumbs on a plate and roll the croquettes in them. Preheat the oven to 200 F. Pour the oil into a skillet to the depth of 1/4-inch and over medium-high heat. Fry the croquettes in batches until golden brown on both sides 5 to 8 minutes. Remove them to paper towels to drain and put in the oven to keep warm. Finish frying the others, then serve them all together.