Place one oven rack near broiler and one close to bottom of oven. Heat oven to 450 ° F. Line a large sheet pan with foil and coat lightly with non-stick spray.
Toss potatoes with 2 Tbsp oil and 1.5 Tbsp seasoning. Arrange, cut side down, on baking sheet. Roast on bottom rack until tender but not yet browned, appx 20-25 minutes.
While potatoes cook, toss onion and asparagus with 2 Tbsp oil and 1 Tbsp seasoning. Mix remaining seasoning (appx 2.5 Tbsp) with 2-3 Tbsp oil and rub all over mushrooms.
Carefully remove pan from oven and use a spatula to move sweet potatoes to edge of pan. Place mushrooms (smooth side down) and veggies in the center in an even layer. Set oven to broil. Place on rack nearest broiler and cook until mushroom is browned, about 4-5 minutes per side. Serve immediately.