Separate the callaloo leaves from any thick, fibrous stems. Thinly slice tender stems, then roughly chop the leaves. Rinse the callaloo very well in a large bowl of cool water, lift it out, and repeat if needed to remove grit. Drain thoroughly and pat dry.
Heat the oil in a large skillet over medium heat. Add the onion and the white parts of the scallions. Cook, stirring often, until softened, about 3 minutes.
Add the garlic, thyme, and optional whole Scotch bonnet. Stir for about 30 seconds, just until the garlic smells fragrant.
Add the chopped tomato and cook until it begins to soften, about 2 minutes.
Add the callaloo a few handfuls at a time, tossing as it wilts so it fits in the skillet. Season with the salt and black pepper.
Cook uncovered, stirring frequently, until the greens are tender and most of their released liquid has evaporated, about 4 to 5 minutes. Remove and discard the thyme sprigs and whole Scotch bonnet before serving.
Stir in the green parts of the scallions and the optional lime juice. Taste and add a little more salt if needed. Serve warm.