Roast Garlic: Chop the top portion off the garlic bulb to expose a bit of the cloves, leaving the paper in tact. Set bulb on a sheet of aluminum foil and drizzle 1 Tbsp olive oil onto it, letting the oil sink into the cloves. Wrap the bulb up in foil and bake for 30 minutes at 400 degrees F (204 C), or until cloves are soft and lightly browned.
Flavor Makers: In a large pot, heat 1 Tbsp olive oil over medium heat then add onion and celery. Cook until celery is bright green and onion is soft, about 5 minutes. Add beans, broth, milk, pepper, and 1 Tbsp of the roasted garlic cloves. Cook over medium heat for 15 minutes.
Blend: Use a slotted spoon to remove and reserve 2 cups of the veggies. Working in batches, or with an immersion blender, blend the remaining soup until creamy. Add the 2 cups of veggies back into the soup.
Serve: Add spinach and cook a few minutes until wilted. Serve warm, optionally topped with parmesan and rosemary.