Slow-roasting fennel and onions is one of the best ways to coax out their inherent sweetness. We start them in a covered baking dish, mixed with olive oil, tahini and seasonings, as well as water that creates steam to help them tenderize. They finish roasting uncovered to cook off any remaining moisture and develop deep, rich, flavorful caramelization. The vegetables are delicious warm or even at room temperature (but hold off adding the mint until ready to serve).