preheat oven to 450°F

In a medium bowl, whisk together vinegar, mustard, 1/4 tsp salt, and 1/4 tsp pepper. Slowly add olive oil, whisking to emulsify. Set aside 1/2 cup dressing to toss with. the greens.

Place mushrooms, stem side up, on a baking sheet. Coat both sides with some of the remaining dressing. Roast mushrooms, brushing with more dressing, and divide among salad bowls.

Cut Portabello's 1/2 " thick, and arrange a few slices over ea serving.