For the vinaigrette:
2 tablespoons white wine vinegar
1 teaspoon Dijon or yellow mustard
1 teaspoon kosher salt, plus more for seasoning
Freshly ground black pepper
1/3 cup olive oil, plus more for brushing
1/4 cup parsley leaves, checked and chopped
For the salad:
4 portobello mushrooms, stems removed
1 yellow squash, sliced
1 small zucchini, sliced
1 orange pepper, cut in strips
1 Vidalia onion, sliced in rings
10 ounces romaine hearts, checked and shredded
Heat your grill pan over medium high heat until hot.
Whisk the vinegar with the mustard, salt, and pepper in a small bowl. Gradually whisk in the oil, starting with a few drops and then adding the rest in a steady stream to make a smooth, slightly thick vinaigrette. Whisk in the parsley and set aside.
Lay the vegetables out on a pan and lightly brush them with olive oil and season with salt and pepper. Place the mushrooms on the grill pan, smooth side down. Grill until juices collect in the cap and mushrooms soften, about 3 minutes. Turn them over and cook until slightly charred around the edges and the center is very tender, about 3 minutes more. Grill zucchini and yellow squash until lightly charred. Put the peppers on the grill pan skin side down and grill until juicy and charred, about 6 minutes. Turn down heat slightly and grill the onions, turning once or twice, until they are tender, about 20 minutes.
Divide the romaine, mushroom caps, tomatoes and onions among 4 plates. Drizzle the vinaigrette over the salads and serve.