How to Make It
Set a very large skillet over moderately high heat until smoking. Add the onion bulbs cut side down and cook until lightly charred, about 4 minutes; transfer to a plate. Add the mushrooms and olive oil to the skillet and cook, stirring occasionally, until lightly browned, 7 minutes.
Add the onion bulbs, garlic and a pinch each of salt and pepper to the skillet. Add the Madeira and cook, stirring, until the mushrooms and onions are tender, about 5 minutes. Stir in the lemon juice and onion tops. Transfer to a serving bowl. Fold in the watercress and season with salt and pepper. Top with watercress flowers, if using, and serve.