Preheat the oven to 375°F (190°C).
Prepare the produce by separating the broccoli into bite-size florets while retaining some of the stems. Slice the carrots into slender rounds, and cut any larger mushrooms in half.
Arrange the broccoli, carrots, and mushrooms in a 9 × 13-inch casserole or lasagna dish. Pour the grapeseed oil over the vegetables, lightly season with salt and black pepper, and toss until the pieces are evenly coated.
Cover the baking dish securely with aluminum foil. Transfer it to the preheated oven and roast for 35–45 minutes, depending on the desired level of tenderness. Check the vegetables by inserting a fork into the carrots and broccoli stems; they should yield easily when properly cooked.
While the vegetables are roasting, place the minced garlic and plant-based butter in a microwave-safe bowl. Microwave for 1 minute, allowing the butter to melt and the garlic to become fragrant.
Add the drained sweet corn kernels and dill to the garlic-butter mixture. Stir thoroughly, then microwave for another 1 minute, or until the mixture is warmed through.
When the vegetables have finished roasting, carefully remove the foil. Spoon the warm garlic, corn, and dill mixture evenly over the roasted vegetables.
Add additional salt and black pepper as necessary. Drizzle the vegetables with 1–2 tablespoons of lemon juice or balsamic vinegar, followed by 1–2 tablespoons of pure maple syrup, adjusting both according to taste.
Gently toss the vegetables and corn mixture together so the seasonings and finishing ingredients are distributed evenly. Serve warm.