Pre-heat oven to 400 degrees F and place oven rack in the top third of the oven.
Spread onion, asparagus, zucchini, and cauliflower on a baking sheet lined with heavy duty aluminum foil (for easy cleanup) pour olive oil over and toss to coat (using your hands is easiest). Season with salt and pepper and roast until tender and slightly browned, about 25 to 35 minutes, checking periodically to make sure nothing burns.
While the vegetables are roasting, make the sun-dried tomato dressing. Through the feed opening of a running blender, add scallions, garlic, sun-dried tomatoes, and the fresh basil leaves. Add the vinegar and blend. Scrape down the sides and blend again until a paste is formed. Then, leaving the blender running, add the olive oil in a slow steady stream allowing the mixture to emulsify. Season the dressing with salt and pepper to taste.
Remove the vegetables from the oven and transfer to a mixing bowl. Stir the cheese into the vegetables to allow it to melt slightly. Fold in peas and grape tomatoes. Mix with enough dressing to coat. (Do not feel compelled to use all of the dressing). Transfer to a serving dish and garnish with basil sprigs. Serve extra dressing on the side.