Arrange oven racks, one in the center and one just below the broiler. Place a rimmed baking sheet on the center rack. Pre-heat oven to 450 degrees F
In a large bowl, combine frozen vegetables, oil, salt, pepper and onion powder; toss to coat
Arrange vegetable mixture on pre-heated baking sheet in a single layer. Return to center rack; roast for 45 minutes
Switch oven to broil; switch baking sheet position to top rack. Optional: dust lightly and evenly with Parmesan. Broil, 1 to 2 minutes, until browned as desired
Remove baking sheet from oven; drizzle evenly with juice of half a lemon and grated lemon zest as desired
Transfer vegetable mixture to a serving dish, serve and enjoy