Step 1

Preheat oven to 400°. Toss brussels sprouts and 3 Tbsp. oil on a rimmed baking sheet; season with salt and pepper. Roast, tossing halfway through, until softened and cut sides are brown, 20–25 minutes.

Step 2

Whisk vinegar, sugar, fish sauce, soy sauce, and remaining 1 Tbsp. oil in a large bowl. Add brussels sprouts and toss to coat; transfer to a platter. Top with pumpkin seeds and rosemary; sprinkle with Aleppo pepper.

Step 3

DO AHEAD: Vinaigrette can be made 1 day ahead. Cover and chill.