Preheat the air fryer: Set to 380°F (193°C). Preheating helps crisp the edges right away.
Prep the vegetables: Trim the stem ends from the Brussels sprouts and remove any wilted leaves.
Halve them lengthwise. Peel and slice the carrots into even pieces, about 1/2 inch thick, so they cook at the same rate.
Toss with seasonings: In a large bowl, combine Brussels sprouts and carrots with olive oil, salt, pepper, garlic powder, and smoked paprika if using. Stir until everything is evenly coated.
Load the basket: Add the vegetables to the air fryer basket in a mostly single layer.
A little overlap is fine, but avoid packing it tightly. If your air fryer is smaller, cook in two batches.
Cook and shake: Air fry for 12 to 16 minutes total, shaking the basket halfway. Start checking at 10 minutes.
You’re looking for deep golden edges on the sprouts and tender carrots you can pierce with a fork.
Finish with brightness: Transfer to a bowl. Add lemon juice and zest. If you like a touch of sweetness, drizzle with honey or maple syrup and toss to coat.
Add toppings and serve: Sprinkle with Parmesan or feta and a pinch of red pepper flakes, if you like.
Taste and adjust salt and pepper. Serve hot.