Preheat a 6-quart air fryer to 390°F for 3 minutes. Pat potato cubes thoroughly dry and toss with 2 teaspoons oil, 1/4 teaspoon salt, paprika and pepper.
Spread potatoes in a loose layer in the basket and cook for 10 minutes, shaking after 5 minutes to expose new faces.
Toss onion with remaining oil and salt. Add to the potatoes, shake gently to distribute and cook for 8 minutes.
Shake again and cook for 5 to 7 minutes more until the potato centers are tender and several faces are golden brown; remove any small onion pieces early if they darken too quickly.
Divide into 4 side portions and serve immediately with both potato cubes and softened onion in each serving.