Position 1 rack in top third and 1 rack in bottom third of oven and preheat to 425°. Brush 2 large rimmed baking sheets with oil; warm sheets in oven 10 minutes.
Quarter fennel bulbs through core. Cut each quarter into 3 wedges; place in large bowl. Add 2 Tbsp oil, sprinkle with salt and pepper, and toss fennel to coat.
Combine carrots and shallots in another large bowl. Add 2 Tbsp oil, sprinkle with salt and pepper, and toss to coat. Spread fennel on 1 hot baking sheet and carrot-shallot mixture on second sheet. Return sheets to oven; roast vegetables 10 minutes. Reverse sheets. Roast vegetables 20 minutes. Turn vegetables over. Roast until tender, 10-15 minutes longer.
Arrange vegetables on platter. Sprinkle with parsley. Garnish with fennel fronds.
