3. Step
While the cauliflower roasts, heat the remaining 1 tablespoon oil in a large, deep skillet over medium-high heat. Add onion and celery; cook, stirring often, until the vegetables soften, about 5 minutes. Stir in 1/2 cup scallions; cook, stirring often, until softened, about 1 minute. Add milk and cream cheese; cook, stirring constantly, until the mixture is thickened and smooth, about 1 minute. Remove from heat.