Wash, trim, and chop the callaloo into ribbons. If using the stems, use only tender ones and scrape off some of the outer skin. Set aside.
Thinly slice the garlic, and chop the onion, scallion, tomato, and Scotch bonnet pepper.
Heat oil in a wide skillet over medium heat. Add onion, garlic, black pepper, and scallion. Reduce heat to low and sauté for 3–4 minutes.
Stir in shrimp (if using precooked, rinse and squeeze out excess moisture). Cook for 3 minutes. For fresh shrimp, cook for 3 minutes, remove, and set aside; return them during the final few minutes of cooking.
Increase heat to medium-high and add the callaloo, tomato, thyme, and Scotch bonnet. Stir to combine; the greens will wilt down.
Add water and simmer uncovered for about 10–12 minutes or until callaloo reaches your desired tenderness.
Taste and adjust salt as needed. Optional: Finish with a splash of lemon juice to balance the flavors. Stir to combine and cook off any excess liquid.
Add reserved shrimp if using fresh. Stir in scallions. Serve hot.