A typical bearnaise recipe has 3 egg yolks and 1 1/2 sticks butter per cup! This lightened version retains the spirit of the original with just 1 egg and 1 tablespoon butter and uses low-fat buttermilk as the base. And switching from steak to hearty portobello mushrooms saves 12 grams of fat. A French chef would garnish the finished dish with watercress sprigs; I’ve expanded watercress’ role from a mere garnish to a nutritious salad that serves as a colorful bed for the portobellos. The bearnaise sauce is also good served cold as a mayonnaise substitute.
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.