- 2 tablespoons minced shallot
- 1 1/2 tablespoons tarragon vinegar or white wine vinegar
- Kosher salt
- 3/4 cup mayonnaise
- 1 tablespoon minced tarragon
- Extra-virgin olive oil, for brushing
- 6 large portobello mushrooms, stems discarded
- One 1-pound beefsteak tomato, cored and cut crosswise into six 1/2-inch-thick slices
- Thinly sliced scallions, for garnish
- In a bowl, whisk the shallot and vinegar with a generous pinch each of salt and pepper; let stand for 5 minutes. Whisk in the mayonnaise, tarragon and 1 1/2 tablespoons of water. Season the béarnaise sauce with salt and pepper.
- Light a grill and brush with oil. Brush the portobello caps with oil and season with salt and pepper. Grill over moderately high heat, turning once, until lightly charred, 6 minutes. Transfer to a plate. Brush the tomato with oil and season with salt and pepper. Grill, turning once, until lightly charred, 3 minutes total. Set the tomatoes on the portobellos.
- Spoon the béarnaise on the tomato slices. Transfer the stacks to the grill and close the grill. Cook over moderate heat until the sauce is just hot, 3 minutes. Transfer to plates, garnish with scallions and serve.
This cheater's béarnaise can be refrigerated overnight.
Lighter, fruity reds are great with vegetarian dishes.