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