Preheat oven to 220°C. Toss the Brussels sprouts with 3 tablespoons of oil and spread them on a rimmed baking sheet; season with salt and pepper. Roast, tossing halfway through, until softened and cut sides are brown for about 20–25 minutes. Take out of the oven and let it cool down a little.
Whisk the vinegar, sugar, fish sauce, soy sauce, and remaining 1 tablespoon oil in a large bowl. Add Brussels sprouts and toss to coat; transfer to a platter. Top with pumpkin seeds and rosemary; sprinkle with pepper flakes.
Recipe by Must Be Yummie at https://mustbeyummie.com/oosterse-spruitensalade/