Preheat the oven to 400°F (200°C). Place the cauliflower florets in a microwave-safe bowl and sprinkle with about 1 teaspoon (5 mL) of water. Cover with plastic wrap and microwave for 3-4 minutes to soften it a little. Pour cauliflower into a colander and drain. Alternatively, if you do not use a microwave, you can place the entire head of cauliflower in a large pot of boiling water and blanch for 2-3 minutes. Remove and drain.
In a large saucepan on medium heat, cook the bacon until crispy. Remove bacon from pan with a slotted spoon and set on paper towels. Add garlic and onion and cook until translucent, about 2 minutes. Lower the heat. Slowly sprinkle in the arrowroot powder as you stir, creating a roux (paste). Whisk in the almond milk slowly until combined. Turn off heat and add the mixture to a large bowl with the cauliflower florets. Stir in the mayonnaise and spices and stir well. Pour into a 9x13-inch baking dish, top with bacon, parsley, and cheese (if using) and bake for 20-25 minutes until it starts to brown on top.