Preheat the oven to 180C/350F/Gas 4.
Brush the undersides of the mushrooms with half of the olive oil, season with salt and freshly ground black pepper and place onto a greased baking tray. Roast for 8-10 minutes, or until the mushrooms begin to soften. Remove from the oven and set aside.
Drain the soaked sun-dried tomatoes and place into the bowl of a food processor. Add the garlic, basil, parsley and the remaining olive oil. Blend to a rough paste, adding more olive oil if necessary to loosen the mixture. Season, to taste, with salt and freshly ground black pepper.
Spread a quarter of the paste inside one of the mushrooms and top with a slice of the goats' cheese. Repeat with the remaining stuffing mixture, mushrooms and goats' cheese. Place the stuffed mushrooms back onto the baking tray and cook for 5-6 minutes, or until the cheese is golden-brown and bubbling.
To serve, mix the watercress and wild rocket together in a bowl and dress with the olive oil and balsamic vinegar. Divide the salad among four serving plates, then place a stuffed mushroom on each plate and serve.