Preheat oven to 350°F.Blanch cauliflower in a large pot of boiling water for 2 minutes.Drain well and set aside.To make the sauce in a skillet heat the olive oil over medium heat.Add garlic and cook for 30 seconds.Add the arrowroot flour and stir until golden.Add the milk and bring to a low simmer, stirring until combined.Stir in the mayonnaise and season with salt and pepper.Add drained cauliflower and toss to combine.Transfer the mixture into a casserole dish.Bake for 30 minutes.Remove from the oven, sprinkle with bacon and parsley and serve.