Preheat oven to 350° F.
Slice the onions and fennel, and cut the mushrooms and carrots in halves or quarters lengthwise. Cut the tough ends off of the asparagus spears. Mince the garlic.
Arrange the vegetables on a baking sheet, and drizzle olive oil and agave over them. Sprinkle with salt and pepper.
Roast for 35-40 minutes, until vegetables are almost tender.