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1
Bring bacon and 8 cups water to boil in a large pot (bacon should be submerged). Attach bay leaf to whole onion by piercing it with cloves; add to pot. Reduce heat to medium. Simmer bacon, turning occasionally, until tender, about 1 hour. Transfer to a plate and let cool. Strain broth through a fine-mesh sieve into a large bowl. Skim off fat from surface and discard.
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2
Combine strained broth and lentils in a large saucepan. If necessary, add more water to cover lentils by 1/2 inch. Bring to a simmer and cook, stirring occasionally, until lentils are just tender but not mushy, 18-20 minutes. Drain lentils, reserving broth. Transfer lentils to a large bowl.
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3
Preheat oven to 375u00b0. Heat oil in a large skillet over medium heat. Add minced onion, carrots, celery, and cayenne; season with salt and pepper. Cook, stirring occasionally, until vegetables are soft and lightly colored, 7-8 minutes. Add garlic, sage, and thyme; stir for 1 minute. Transfer to bowl with lentils.
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4
Cut skin from cooled bacon; discard skin. Cut bacon crosswise into 1/3-inch-thick slices. Add to lentils; toss mixture gently until well combined. Season with salt and pepper. Transfer mixture to a 3-qt. baking dish.
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5
Distribute sausage evenly over lentil mixture, gently pushing into lentils. Moisten lentils with reserved broth to barely cover (2-2 1/2 cups; add water if needed).
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6
Combine breadcrumbs with butter in a medium bowl; season lightly with salt and pepper. Toss to combine. Sprinkle evenly over lentil mixture; cover dish tightly with foil and place on a rimmed baking sheet.
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7
Bake cassoulet for 45 minutes. Remove foil; bake until breadcrumbs are golden and juices are simmering, 25-30 minutes. Let cassoulet rest for 15 minutes before serving. Sprinkle with parsley and chives; serve with Dijon mustard and cornichons, if desired.
