Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius). Grease four nine- by 13-inch (20- by 30-centimeter) aluminum pans generously with cooking spray.

Place potatoes, mushrooms, zucchini, and asparagus in the pans, each in a single layer. Drizzle the veggies generously with oil and sprinkle salt, pepper, and 1/4 teaspoon Italian seasoning over each pan. Add one tablespoon balsamic vinegar to the mushrooms and 1/2 tablespoon cornstarch, 1/2 teaspoon paprika, and 1/4 teaspoon garlic powder to the potatoes. Toss all vegetables until evenly coated. Make sure to keep them in a single layer and slightly spread apart.

Place pans into preheated oven, with the asparagus and mushrooms on the lower rack and potatoes and zucchini on the upper one. Roast for 20–25 minutes. Remove the asparagus when lightly browned. Toss the rest of the vegetables and roast them for an additional 10–15 minutes.

Drain juices from the mushrooms. Cut asparagus spears into one- and- a- half-inch (three- to four-centimeter) pieces.

While the vegetables are roasting, combine all dressing ingredients and whisk well until emulsified. Set aside.

Place lettuce and grape tomatoes into a large bowl. Add warm roasted veggies and toss gently. Top salad with crumbled feta cheese and sliced scallions. Drizzle with dressing, toss lightly, and serve immediately.