Preheat oven to 450ºF. Add Brussels sprouts to a large rimmed baking sheet, and toss with avocado oil, ½ tsp. salt, and black pepper. Arrange cut side-down, avoiding any overlap, and roast for 20 minutes.
Meanwhile, combine mayonnaise, sriracha, vinegar, sugar, garlic, and remaining ¼ tsp. salt in a small bowl; mix well to combine.
Transfer roasted Brussels sprouts to a bowl and toss with half of the sauce. Drizzle remaining sauce overtop, and garnish with green onion and sesame seeds.
