Make breadcrumbs: Rip up pieces of bread and put in food processor. Pulse until grainy. Add 2 teaspoons of garlic plus tablespoons of olive oil. Pulse 3-5 times until incorporated. In a large skillet on medium heat, toast the breadcrumbs until crispy, about 15 minutes.
Cook pasta: Put a large pot of water on high. Add 3 tablespoons salt. When boiling, cook pasta according to instructions for al dente results. KEEP a cup of the pasta water to use in the sauce.
Make sauce: Meanwhile, remove the breadcrumbs from the skillet and set aside. Use the same pan to cook the onions and cauliflower in about 3 tablespoons of olive oil, basil and 1 teaspoon of salt until soft, about 5 minutes. Pour the mixture into the food processor, add cheese and grind on low until a paste forms. Add a little of the pasta cooking water at a time until it all comes together in a thick sauce.
Combine everything: Pour the cheese and cauliflower sauce over the pasta and stir. Sprinkle with breadcrumbs and serve.