In a small bowl, whisk together the olive oil, vinegar, Italian seasoning, Dijon mustard, garlic, salt, sugar, and pepper. In a large bowl, combine the cauliflower, cherry tomatoes, cherry pepper halves, zucchini, carrots, and onion.
Pour the dressing mixture over the vegetables, tossing gently to coat. Cover and refrigerate for at least 2 hours or up to 24 hours, stirring occasionally. Serve with a slotted spoon.