Portobello 'Steaks' with Bearnaise Sauce

Ingredients

 * ¼ cup dry white wine
 * 2 tbsp white wine vinegar
 * ¼ cup finely chopped shallots (2 small)
 * 1 tbsp chopped fresh tarragon or 1 tsp dried
 * ½ tsp whole black peppercorns, crushed
 * 1 large egg
 * 1 tbsp cornstarch
 * ⅔ cup buttermilk
 * 1 tbsp butter
 * 1 tsp Dijon mustard
 * ¼ tsp salt
 * 3 tbsp balsamic vinegar
 * 3 tbsp extra-virgin olive oil
 * ½ tsp salt
 * ¼ tsp freshly ground pepper
 * 12 medium portobello mushroom caps (1½ lbs total)
 * 8 cups watercress (about 4 bunches), large stems trimmed
 * 1 cup cherry tomatoes, halved
 * ½ cup slivered red onion

Directions

 * 1) Make sauce: in small saucepan, combine wine, vinegar, shallots, tarragon and peppercorns and bring to a simmer over medium heat.
 * 2) Cook until most of the liquid has evaporated, 2 to 4 minutes.
 * 3) Remove from heat and set aside.
 * 4) In medium bowl, whisk together egg and cornstarch, then gradually whisk in buttermilk until mixture is smooth.
 * 5) Add to shallot mixture and return saucepan to medium heat.
 * 6) Cook, whisking constantly, until mixture simmers and thickens slightly, about 2 minutes.
 * 7) Whisk in butter, mustard and salt.
 * 8) To keep sauce warm, place saucepan in larger pan of barely simmering water.
 * 9) Preheat broiler.
 * 10) In small bowl, whisk together vinegar, oil, salt and pepper.
 * 11) Using small spoon, scrape away black gills inside mushroom caps.
 * 12) Brush half the balsamic mixture over both sides of caps.
 * 13) Place mushrooms, cavity-side down, on baking sheet or broiler pan and broil until tender and browned, 4 to 5 minutes per side.
 * 14) Meanwhile, in large bowl, combine watercress, tomatoes and onion.
 * 15) Add remaining balsamic mixture and toss to coat.
 * 16) Divide salad among plates and top each with 3 portobello "steaks," cavity-side up.
 * 17) Spoon dollop of bearnaise sauce onto each and serve at once.