Preheat the oven to 400°F.
Together in a large bowl, mix the olive oil and about 1/4 teaspoon salt. Toss the cauliflower in the oil until well coated. Spread the cauliflower on a rimmed baking sheet and roast until beginning to brown and just tender, 22 to 25 minutes.
Meanwhile, in a small bowl, mix together the ingredients for the cheese sauce: yogurt, cheese, milk, mayonnaise, garlic, chives, salt and pepper. Cover and chill until ready to serve.
In another small bowl, mix together the melted butter and Sriracha.
Using tongs or a spatula, put the roasted cauliflower in a large bowl. Drizzle with the Sriracha-butter sauce and gently toss to coat. Return to the baking sheet in a single layer and roast until the sauce is bubbling and the cauliflower is browned around the edges, about 6 to 8 minutes.
Serve hot with the cheese sauce. Garnish with sticks of celery, if desired.