Add the avocado oil to a large non-stick skillet and heat to medium-high. Add the sliced onions to the large skillet, sprinkle with sea salt and cover with a lid. Cook covered for about 3 to 5 minutes, until the onions have softened and are beginning to turn golden-brown. Remove the lid and cook for another 5 minutes, stirring occasionally. Reduce to medium heat or medium-low heat and continue cooking, stirring occasionally, until the onions are deeply golden-brown and caramelized, about 50 to 60 minutes.

Once the onions have caramelized, stir in the almond flour until well-combined.

NOTE: Because this process takes the longest, I recommend starting it first and preparing the rest of the recipe as the onions are caramelized.

Preheat the oven to 375 degrees F and lightly oil a large casserole dish or baking dish.

Wash the broccoli and cauliflower well and pat dry. Remove the stems and chop the head of broccoli and the cauliflower into florets. Transfer the broccoli florets and cauliflower florets to the prepared casserole dish and drizzle with 1 tablespoon of avocado oil. Use your hands to toss everything together until the florets have a light coat of oil. Lightly sprinkle with sea salt.

Roast the broccoli and cauliflower in the preheated oven for 35 minutes, until softened and they begin turning golden-brown.

While the veggies are roasting, prepare the sauce.

Add the garlic, full-fat canned coconut milk, cider vinegar and sea salt to a saucepan and bring to a full boil.

Remove the sauce from the heat and allow it to cool for a few minutes. Whisk the egg yolks into the sauce and stir well. Place the skillet back on the stove top and heat over medium-high heat, stirring constantly, until the mixture is steamy and reaches a controlled boil.

Stir in the gluten-free all-purpose flour and continue cooking the sauce for about 3 to 5 more minutes, or until it is very thick and resembles the consistency of custard. Taste the sauce for flavor and add more sea salt and/or cider vinegar to taste.

Pour the sauce over roasted veggies and stir until everything is well-coated in sauce.

Distribute the caramelized onions over the casserole evenly.

Bake at 375 degrees F for 10 minutes. 

For crispier onions, place the oven on the high broil setting and move the casserole to the second-to-the-top shelf of the oven. Broil for 1 to 3 minutes, until golden-brown and slightly crispy.

Serve the casserole immediately with your main dish or store covered tightly in plastic wrap for up to 5 days prior to serving it. Reheat in the oven at 350 degrees for 20 minutes if you're making this ahead of time.