Preheat oven to 425 degrees F and line one or two rimmed baking sheets with a silicone baking mat.
Wash and dry veggies.
Cut cauliflower/broccoli into florets and cut onion and peppers into large chunks.
Place prepared veggies onto prepared baking sheet (s).
Toss with olive oil, apple cider vinegar, garlic, salt, and pepper.
Be sure to spread the veggies out in a single layer with space in between. You don't want them crowded or they'll steam instead of roast! If necessary, use two baking sheets instead of one.
Roast, stirring every 10 minutes, for 20-30 minutes, or until vegetables are cooked to your liking.