2 large portabella mushrooms 1/4 onion, chopped 1 teaspoon garlic, minced 1-1/2 teaspoon olive oil 2 teaspoons gf worcestershire sauce Salt and freshly ground black pepper 2 Tablespoons balsamic vinegar 3/4 cup fresh spinach, 1/4 cup finely chopped 1/4 teaspoon dried tarragon 1/2 cup heavy cream 1/2 teaspoon cornstarch 1 Tablespoon gf stone ground mustard
Rinse and drain spinach. Chop 1/4 cup and set aside.
Trim off and discard any mushroom stems to make ends flush with interior of caps; rinse caps and pat dry.
In a small bowl, mix olive oil and 1-1/2 teaspoon gf worcestershire sauce. Rub mixture evenly over both sides of mushroom caps. Sprinkle both sides with salt and pepper.
Lay caps, gill side down, on a broiler pan.
Broil about 4" from heat until juice starts to drip from mushroom caps, 4 to 5 minutes. Turn caps over and broil until juice in caps bubbles and mushrooms are flexible, 5 to 7 minutes longer.
Remove from broiler and loosely tent with foil to keep warm.
Meanwhile, in a medium skillet combine onion, garlic, vinegar, and tarragon. Boil over high heat, stirring often, until all liquid has evaporated, about 1 minute.
In small bowl, mix cornstarch and 1 tablespoon water until smooth; whisk in cream, mustard and remaining 1/4 teaspoon gf worcestershire sauce. Add to shallot mixture and stir until boiling. Adjust seasonings with salt and pepper to taste.
Divide fresh spinach leaves on plates. Top with mushroom caps. Spoon sauce equally over mushrooms and sprinkle with chopped spinach.
Serve this with mashed potatoes and a mixed greens salad with a light vinaigrette.