Bring a large pot of water to boil, add macaroni to pot and cook according to package directions or until al dente. Drain macaroni, saving 2/3 cup (150 mL) pasta cooking water.
Puree cauliflower and reserved pasta cooking water in a food processor until smooth. Season with salt and pepper to taste. Reserve.
Preheat oven to 350 ° F (180 °C).
Heat olive oil in a skillet over medium heat. Add onions and garlic and sauté for 1 minute. Add zucchini and cook until zucchini softens, about 5 minutes. Add zucchini mixture to pasta.
Reduce heat to low and simmer for 5 minutes to combine flavours. Season well with salt and pepper. Remove from heat and use an immersion blender or food processor to puree sauce until silky smooth.
Add sauce to pasta mixture and toss to coat. Taste for seasoning.
Oil a 9-by-13-inch (23-by-32-cm) baking dish or other gratin dish. Pile in dairy-free creamy macaroni mixture, sprinkle on breadcrumbs and bake for 30 minutes or until dish is hot and sauce is bubbling. If the top has not browned, then broil top for 2 minutes or until a slight crust forms.