Preheat oven to 425°F. Spray an extra-large, rimmed baking sheet, or 2 smaller ones, with non-stick cooking spray.
Place Brussels sprouts, green beans, broccoli, asparagus, mushrooms, fennel, onion and garlic in a large bowl.
In a small bowl, whisk together the olive oil, vinegar and agave. Pour mixture over vegetables and toss to coat.
Spread vegetables evenly in a single layer on prepared pan(s). Season with salt and pepper. Place lemon halves on the pan.
Roast 20-22 minutes in preheated oven until vegetables are softened and caramelized, stirring once halfway through. When cool enough to handle, squeeze the lemons over the roasted vegetables.
Sprinkle with parmesan cheese and walnuts (optional), basil and parsley. Stir to combine. Season to taste with salt and pepper.
Serve in a large bowl or platter.
Can be made 2 hours ahead. Cover and let stand at room temperature. Rewarm at 350°F for 10-15 minutes.
