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.