Preheat oven to 375°F. Place washed and dried cauliflower on a parchment-lined baking sheet. Drizzle with olive oil and sprinkle with a bit of sea salt and pepper. Roast in oven for 30-45 minutes (turning halfway through) until cauliflower is tender and pieces start turning brown and carmelizing.
Meanwhile, in a large pot, prepare the pasta according to the directions (we like ours on the "al dente" side). Drain, reserving a bit of the liquid, and transfer the pasta and liquid to a bowl. Set aside.
In the same pot, over medium heat, add 2 tablespoons butter, plus the onions. Cook until onions are tender, about 10 minutes.
Add the garlic and remaining butter. Whisk in the brown rice flour and cook for a minute. Slowly add the milk and mustard, whisking continuously. Turn down the heat to medium-low and let simmer until it starts to thicken a bit (about 2 minutes), stirring often.
Add half the cheddar and stir until it comes together. Turn off heat and carefully mix in the pasta and cauliflower. Season with salt and pepper to taste (remember the cheese will add salt, so go easy).
Transfer to a skillet or casserole dish and top with remaining cheese. Bake for 8-10 minutes at 375°F and broil at the end to brown the cheese to your liking.
Top with basil and serve.