Preheat oven to 450 degrees. In a medium bowl, whisk together vinegar, mustard, 1/4 teaspoon salt, and 1/4 teaspoon 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 every 5 minutes, until they are tender, about 15 minutes.
Toss endive, mesclun, onion, and cheese with reserved dressing, and divide among salad bowls. Cut portobellos 1/2 inch thick, and arrange a few slices over each serving.
Found this recipe as we went back through saved copies of Everyday food, and we LOVED It. My husband hates mustard, so we left that out, and it still worked fine. The combination of salty, bitter and crunch with the richness of the mushrooms all came together beautifully.