Pour the olive oil mixture over the cauliflower and grapes and toss well to coat. Spread on the prepared sheet pan and roast in the oven for 25 - 30 mins until browned and the grapes have burst.
Yogurt Sauce:
Whisk all of the ingredients together until smooth.
To Serve:
Drizzle the roasted cauliflower and grapes with the yogurt sauce and sprinkle with fresh mint if desired.