Preheat the oven to 425 degrees Fahrenheit.
Cut the cauliflower into medium sized florets and set aside.
In a medium mixing bowl, whisk together the flour, milk, garlic, onion, salt, pepper and cornstarch until smooth.
In a second bowl, add the breadcrumbs.
Take one piece of cauliflower, dip it in the batter, let the excess drip off, then roll into the breadcrumbs.
Place on a baking sheet lined with parchment paper. You can just add the breadcrumbs to the same bowl as the batter and mix together, but I prefer the texture when it's done separately.
Repeat until all cauliflower has been used. The batter may thicken a bit as it sits, if it gets too thick, just add a splash of milk and whisk.
Bake for 25 minutes or until the cauliflower has browned slightly. After 25 minutes, I usually turn the oven off and let it crisp up for an additional 5 minutes, but it should be done at 25 depending on how large the florets were.
Remove from the oven and let cool for 5 minutes.
Whisk together all sauce ingredients. Combine the cauliflower with the sauce and serve immediately.