Portobello 'Steaks' with Bearnaise Sauce

Ingredients

• dry white wine
  ¼ cup
• white wine vinegar
  2 tbsp
• finely chopped shallots (2 small)
  ¼ cup
• chopped fresh tarragon or 1 tsp dried
  1 tbsp
• whole black peppercorns

  crushed
  ½ tsp
• egg
  1 large
• cornstarch
  1 tbsp
• buttermilk
  ⅔ cup
• butter
  1 tbsp
• Dijon mustard
  1 tsp
• salt
  ¼ tsp
• balsamic vinegar
  3 tbsp
• extra-virgin olive oil
  3 tbsp
• salt
  ½ tsp
• freshly ground pepper
  ¼ tsp
• portobello mushroom caps (1½ lbs total)
  12 medium
• watercress (about 4 bunches)

  large stems trimmed
  8 cups
• cherry tomatoes

  halved
  1 cup
• slivered red onion
  ½ cup