Step 1: Prep the Vegetables: Break the broccoli into small florets. Trim the zucchini and cut it into 2cm cubes, skin on. Slice the carrot on the diagonal into thick rounds (the bias cut increases the surface that meets the heat). Snap the woody ends off the asparagus and cut into pieces. Keep all the pieces close to the same size so they cook at the same rate.
Step 2: Toss in Herb Oil: Whisk the olive oil with the garlic powder, oregano, black pepper, and paprika. Pour it over the vegetables and toss with your hands until every piece wears a thin, even film. The oil is the medium for the crisp shell — but don't overdo it, or the veg turns soft.
Step 3: Load the Basket: Spread the vegetables in a single layer with no overlapping (overlap means steam, not crisp). Set the air fryer to 190°C (375°F) for 12 minutes.
Step 4: Shake at the Halfway Mark: Pull the basket out at 6 minutes and shake or flip so all sides meet the hot air. The broccoli edges should be browning and the carrot should yield to a fork.
Step 5: Finish and Serve: Tip the vegetables onto a plate, scatter the optional salt substitute, and squeeze a little lemon over the top to brighten. Eat them hot — that's when they're crispest.