Preheat the oven to 400 degrees F.
For the cheese sauce: Whisk together the sour cream, blue cheese, milk, mayonnaise, 1/8 teaspoon salt and a few grinds of pepper in a small bowl. Cover and refrigerate until chilled, about 30 minutes
For the Buffalo cauliflower: Meanwhile, microwave the butter in a small microwave-safe bowl on high until melted. Whisk in the hot sauce and lemon juice and set aside.
Mix olive oil, 1/4 teaspoon salt and 1/2 cup water in a large bowl. Add the cauliflower and toss until well coated. Spread the cauliflower on a rimmed baking sheet and roast until beginning to brown and just tender, 20 to 25 minutes. Whisk the hot sauce mixture again, drizzle over the cauliflower and toss with tongs to coat. Roast the cauliflower until the sauce is bubbling and browned around the edges, 5 to 7 minutes more. Serve hot with the cheese sauce.
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