1/2 lb. large white mushrooms, stems removed
1 tbsp. fresh lemon juice
1 tbsp. Dijon style mustard
1/2 tsp. salt
1 tsp. freshly ground pepper
1/4 c. plus 1 1/2 tsp. peanut oil
1 1/2 c. walnut pieces (about 6 oz.)
1 1/2 tbsp. unsalted butter, melted
1/4 c. plus 2 tbsp. extra virgin olive oil
1 1/2 tbsp. red wine vinegar
9 c. mache or a mixture of Boston lettuce, romaine lettuce and arugula

Preheat the oven to 400 degrees. Slice the mushroom caps horizontally about 1/4 inch thick. Stack the slices and cut them into 1/4 inch sticks. In a medium bowl, combine the lemon juice, mustard, 1/4 teaspoon of the salt and 1/2 teaspoon of pepper. With a fork, beat in the peanut oil. Add the mushrooms and toss to combine. The mushrooms can be prepared up to 3 hours ahead. Set aside at room temperature.

On a baking sheet, toss the walnuts with the melted butter. Sprinkle with a pinch of salt and bake until golden brown, about 10 minutes. The walnuts can be prepared up to 10 hours ahead. Let cool, then store in an airtight container.

In a medium bowl, stir together the olive oil, vinegar and the remaining 1/4 teaspoon salt and 1/2 teaspoon pepper. The dressing can be prepared up to 1 day ahead. let return to room temperature before using.

Just before serving, put the salad greens in a large bowl and toss with the dressing. Arrange the salad on 12 serving plates. Spoon equal amounts of the mushrooms in the center and scatter the walnuts over the top.