Heat butter and oil in a large pot. Add sausage and cook through until browned and charred on the outside. Remove and set aside.
Add onion, carrot, celery, and ¼ teaspoon salt to the pot. Cook, stirring frequently, until vegetables are softened but not browned, about 5 minutes.
Add garlic and sauté an additional minute. Add broth, rosemary, red pepper flakes, bay leaves and half the beans. Bring to a boil, reduce to a simmer, and cook for 10 minutes.
Transfer the other can of beans to a blender jar along with enough liquid to blend to a smooth puree. Add puree back to the pot of beans. Add the sausage and a few more grinds of salt. Cook an additional 5 minutes.
Add spinach and continue cooking until completely wilted, about 5 minutes. Remove bay leaf. Season to taste with a few more grinds of salt and pepper. Stir in lemon juice. Ladle into bowls.
NOTE: Adding salt throughout really helped it taste amamazing at the end! Must do!