Put a baking sheet in the oven and heat to 500 degrees.
While oven is preheating cut the stalk off of the head of broccoli, remove skin and chop in half lengthwise. Roughly chop the head of broccoli and cauliflower and put in a mixing bowl. Toss with the olive oil, vinegar, salt, pepper, and fennel seeds.
When the oven is heated remove the baking sheet and place the broccoli and cauliflower on it so they are in rows, not piled on top of each other. Place back in the oven for 10-15 minutes until nicely browned and tender.
Try This: serve alongside barbequed chicken or steak with a garden salad. You could copped up some onion and garlic and toss it in with the mix. Instead of the fennel seeds you could try other spices like cumin or dried herbs.