PREPARATION:

Portobello Burgers:

1. In a small bowl, whisk together the vinegar, oil, garlic, and rosemary. Place the mushrooms in a large re-sealable plastic bag, or a baking dish, and cover with the marinade. Marinate for at least 1 hour and up to 2 hours.
2. Prepare the grill.
3. Drizzle onions with olive oil and place on grill. Cook until softened and slightly charred.
4. Remove the mushrooms from the marinade and pat dry. Season both sides with salt and pepper, and place gill sides-down on the grill.
5. Cook for 5 minutes, then turn. Arrange the cheese on the gills of each mushroom and continue to cook until tender, about 6 to 8 minutes.
6. Open the rolls and lightly toast on the grill, insides down.
7. Place 1 grilled mushroom inside each bun and top with grilled onions. Garnish each, to taste, with aioli.
8. Divide the watercress among the 4 buns and serve immediately.

Lemon Aioli:

1. While the food processor is running, drop in garlic cloves. Add the salt, eggs and lemon juice. Pulse to incorporate. Add the cayenne. Slowly drizzle in the oil, a little at a time, until all the oil is incorporated and the mixture emulsifies. Keep refrigerated until ready to use. (Use within 24 hours.)