Heat the oven to 190°C (Gas Mark 5 / 375°F). Toss the asparagus in the olive oil with some salt and pepper, and spread it out on a baking tray. Roast for around 20 minutes, until tender. Set aside.
Meanwhile, add the watercress to a mini food processor, and blitz until well chopped. Add the goat's cheese, and blitz again until a smooth spread is formed. Season to taste.
Spread each crostini with a little of the goat's cheese spread, and top with a piece of asparagus, a very thin slice of radish, and a few small watercress leaves to garnish.
Serve lightly chilled.