Clean and cut cauliflower into medium-sized florets. Toss the florets in oil, salt, and pepper. Bake in a preheated oven at 400 F for 15-20 minutes. Cauliflowers should be just cooked and tender.
While the cauliflower florets are in the oven, make the sauce.
Melt butter in a saucepan. Saute sliced onion until softens(no need to brown). Stir in flour for a minute until the raw smell is gone.
Gradually add milk while stirring with the other hand. Keep adding milk until the mixture is smooth. Add salt, pepper, and ground nutmeg.
Stir in shredded cheddar and gruyere until the cream sauce is smooth.
Once the cauliflower are done, add to the above cheese sauce. Mix well and transfer to a 2-3 qt baking dish. Sprinkle with breadcrumb parmesan mixture and bake for 15-20 minutes until the sauce is bubbly around the edges and the breadcrumb is crispy golden brown in colour.
Sprinkle Let it rest for 5-6 minutes before serving.