In a heavy, 5- to 6-qt. pot or dutch oven, melt the butter over medium-high heat. Add the sausage and cook, stirring occasionally, until lightly browned, about 5 minutes. Lower the heat to medium, add the onion and cook until soft, scraping up the browned bits from the bottom of the pan, 6 to 8 minutes. Add the celery root and carrots and season with salt and pepper. Cook until the vegetables are tender, about 10 minutes.
Meanwhile, mash 1 cup beans and set aside. Add the curry paste to the pot and cook, stirring, for 30 seconds. Stir in the beef stock, then add the mashed beans and remaining whole beans and bring to a boil. Simmer for 15 minutes. Add the spinach and let wilt. Serve hot.