Heat oil in a large pot over medium heat. Add spinach, onion, and celery. Cover and cook, stirring a few times, until softened, 5 to 10 minutes. Add broth, dark red kidney beans, cannellini beans, diced tomatoes, tomato paste, garlic powder, parsley, oregano, red pepper flakes, salt, and black pepper. Heat to a boil, about 30 minutes. Boil for 1 to 2 minutes.
Place a slice of bread in the bottom of a bowl. Ladle soup on top. Repeat with remaining bread.
