Meanwhile, put the olive oil into a large pot over medium-high heat. Crumble in the sausage and cook, stirring now and then, until all of the fat is rendered and the meat is crisp and browned, about 15 minutes. Add the onions, celery, garlic, and 1 tsp salt to the pot and turn down the heat to medium. Cook the vegetables, stirring now and then, until slightly softened, about 10 minutes. Add the sage and stock and turn the heat to high. Once it is at a boil, turn off the heat. Stir in the spinach, parsley, and reserved bread. Taste the mixture and season with salt if more is needed. Add the eggs and give everything one good final stir.