Preheat oven to 450° F, set top rack to highest spot. Line a rimmed sheet pan with foil and coat lightly with oil. Add potatoes, 2 Tbsp oil and 1½ Tbsp seasoning to a bowl, toss to coat. Spread potatoes evenly on the pan and roast until tender, about 20 minutes. In a bowl, add vegetables, 4 Tbsp oil and the remaining seasoning. Coat evenly. When potatoes are tender, remove from oven and set to broil. Move potatoes to outer edge of pan. Place onion and asparagus in the middle, leaving space between. Place the mushrooms smooth side down. Transfer pan to oven. Broil until mushrooms are browned, 5 minutes. Flip mushrooms and stir vegetables, broil for another 5 minutes.
