Preheat the oven to 400 degrees Fahrenheit and line a shallow oven tray with baking paper. Slice the fennel into half inch thick strips. Arrange the fennel, cauliflower and broccoli florets across the oven tray.
Sprinkle the rosemary, thyme, garlic powder, salt and pepper over the vegetables. Drizzle over the olive oil. Use your hands to rub the oil and seasonings into the vegetables.
Roughly dice the butter and dot around the vegetables. Sprinkle over the lemon zest. Transfer to the oven to bake for 20-25 minutes, or until all the vegetables are tender and golden, turning halfway through.