In a large stockpot over medium-high heat, bring 2 cups of water to a boil. Add the okra, onion, scallions, spinach, milk, salt, pepper, allspice, garlic, thyme, and cloves. Stir to combine and cover the pot. Cook 7 to 10 minutes, until the spinach is wilted.
Add the whole pepper, cover the pot, and reduce the heat to medium-low. Simmer for 20 minutes. Remove and discard the pepper. Stir in the butter.
Using an immersion blender, purée the mixture until smooth. (If you don't have an immersion blender, feel free to use a regular blender, working in batches.)
Serve hot over white rice. Or serve as an alternative to spinach dip. To serve as a soup, add 2 to 3 cups of boiling water after the callaloo has been puréed (adjust the salt and pepper accordingly).