Preheat oven to 400º F. On a baking sheet, toss the butternut squash, peppers, brussels sprouts, dried oregano, salt, pepper and olive oil with your hands until the veggies are evenly coated. Spread the vegetables out into one even layer and roast for 40 minutes.
To make the dressing, place the avocado, garlic, lime juice, olive oil, salt and pepper in a blender or food processor and blend until the mixture is light and creamy.
To assemble the salad, place a handful of mixed greens on a plate and top with the roasted veggies. Drizzle avocado dressing over top and serve.