Heat oven to 425°F. In large bowl, toss fennel slices, bell peppers and onion with oil. Place in ungreased 15x10x1-inch baking pan. Bake at 425°F. for 5 minutes.
Remove pan from oven. Add snapper fillets to pan; sprinkle with salt and pepper. Surround fish with partially baked vegetables.
Return to oven; bake an additional 10 to 15 minutes or until fish flakes easily with fork. Garnish fish with fennel leaves.