Drain the cauliflower and add to a high speed blender with the onion, garlic, milk, parmesan, dijon mustard, lemon juice, red pepper flakes, sea salt, and black pepper. Blend on high until smooth, creamy, and warm, scraping down the sides every so often. This can take anywhere from 3 - 5 minutes.
1 cup milk, ⅓ cup parmesan, 1 tsp dijon mustard, 1 Tbsp lemon juice, ¼ tsp red pepper flakes, ¼ tsp sea salt, ⅛ tsp black pepper