Preheat the air fryer: Set to 400°F (200°C) for 3–5 minutes. A hot basket helps the vegetables crisp quickly.
Prep the vegetables: Rinse and dry well. Halve the Brussels sprouts through the core so they hold together.
Halve or quarter potatoes so pieces are similar in size to the sprouts.
Season: In a large bowl, toss potatoes and Brussels sprouts with oil, salt, pepper, garlic powder, paprika, and onion powder if using. Coat evenly. Add red pepper flakes for a little heat if you like.
Load the basket: Spread the vegetables in a single layer.
Some overlap is okay, but avoid a packed mound. If your air fryer is small, cook in two batches for best crisping.
Cook: Air fry at 400°F (200°C) for 14–18 minutes, shaking the basket or stirring halfway. Start checking at 12 minutes.
You want deep golden edges and fork-tender centers.
Finish: Transfer to a bowl. Toss with lemon zest and a sprinkle of Parmesan if you like. Taste and adjust salt and pepper.
Serve: Garnish with chopped herbs.
Serve hot with your main or enjoy as a snack with your favorite dipping sauce.