In a medium skillet, heat 2 tablespoons olive oil over medium heat. Add the onion and cook until softened, about 8 minutes. Stir in one-third of the garlic. Add the chopped watercress and cook until wilted; season with salt. Let cool.
Meanwhile, in a small, dry skillet, toast the curry powder and remaining garlic over medium-low heat until fragrant, about 2 minutes. Transfer to a small bowl; let cool. Stir in the mayonnaise.
In a bowl, combine the lamb with 3/4 teaspoon salt and 1 teaspoon pepper and shape into 4 mounds. Place 1 mound in your hand, press one-quarter of the watercress mixture into the mound and shape the lamb around it. Gently press to form a patty. Repeat with the remaining mounds and watercress.
In a large skillet, heat the remaining 2 tablespoons olive oil over medium heat. Add the patties and cook, turning once, for 12 minutes.
Spread some curry mayonnaise on each roll bottom; top with a patty, more curry mayonnaise, some radishes, watercress sprigs and a roll top.