1. Make the stock: Melt butter in a medium-sized pot over medium-high heat. Add shrimp shells and cook for 5 minutes, stirring occasionally. Add all other ingredients except chicken stock and cook until vegetables are soft. Add stock and cook at a medium simmer for 45 minutes. Strain through a fine mesh strainer and reserve.
2. Make the callaloo: In a heavy-bottom pot, melt butter over medium heat and add onion and garlic. Cook until soft. Add thyme and cayenne; cook for 2 minutes. Add shrimp stock and ham hock. Simmer for 1 ½ hours, or until ham hock is tender. Add okra and cook for an additional 20 minutes. Add spinach and cook for 20 minutes more. Taste for salt and adjust.
3. Make the shrimp: Lay out shrimp in a single layer on a sheet tray and season with curry powder and 1 teaspoon of salt. Heat olive oil in a large skillet over high heat until very hot and carefully add shrimp in a single layer. Working in batches if necessary, sear shrimp until they are light golden brown on one side, then add callaloo. Cook for 2 minutes or until shrimp are just cooked through. Serve on its own or with rice.