Preheat oven to 375°F and grease a 8x11 or 9x13 casserole dish.
Chop cauliflower into florets and place in a large pot. Add enough water to cover the cauliflower and heat until water is boiling. Reduce heat to medium-low and simmer until the cauliflower is tender, but still firm (about 4-5 minutes). Drain, pat dry and set aside.
While cauliflower is cooking, add butter to a large saucepan and heat over medium heat. Add chopped onion and garlic and sauté until tender and fragrant, about 3-4 minutes.
Add flour, salt, pepper and nutmeg. Stir to combine. Slowly add almond milk and bring mixture to a boil, whisking constantly until mixture is smooth and thick,
Remove from heat and add ¾ cup gruyere cheese and ¼ cup parmesan, mixing until fully melted and smooth.
Add cauliflower to prepared baking dish then pour sauce evenly over cauliflower.
Top with remaining cheese and breadcrumbs or almond meal.
Bake for 25-30 minutes until bubbly and top begins to brown. Broil for 2-3 min to brown the top more.
Let cool for a few minutes then serve warm topped with fresh parsley.