Remove the sausage from its casing and tear it by hand into bite-size pieces. Heat 1 Tbs. of the olive oil in a 4- or 5-quart heavy pot or Dutch oven over medium heat. Add the sausage and cook, stirring occasionally, until lightly browned, about 5 minutes. With a slotted spoon, transfer the sausage to a plate, leaving any rendered fat in the pot.
Add the remaining 1 Tbs. olive oil to the pot, increase the heat to medium high, and add the onion. Cook, stirring frequently, until fragrant and beginning to soften, about 2 minutes. Add the carrot and celery and cook, stirring frequently, until they begin to soften and brown, about 2 minutes more. Be sure to scrape any brown bits from the bottom of the pan. Stir in the garlic, pepper flakes, 1/2 tsp. salt, and 1/4 tsp. pepper and cook, stirring, until the garlic is fragrant, about 1 minute more. Add the chicken broth and bring to a boil over high heat.
When the broth reaches a boil, reduce the heat to medium, add the sausage along with any collected juices, and half the beans. Mash the remaining beans with a fork or wooden spoon and add them to the pot, stirring to distribute. Stir in the kale, adjust the heat as necessary to maintain a gentle simmer, and simmer until the kale is tender, 15 to 20 minutes. Stir in the lemon juice and lemon zest (if using) and season to taste with salt and pepper.