Preheat a 6-quart air fryer to 390°F for 3 minutes. Thoroughly dry potato cubes; toss with 2 teaspoons oil, 1/4 teaspoon salt, paprika, and pepper.
Spread loosely in basket; cook 10 minutes, shaking after 5 minutes to expose new faces.
Toss onion with remaining oil and salt. Add to potatoes; gently shake to distribute and cook 8 minutes.
Shake; cook 5 to 7 minutes more until centers tender and several faces golden brown. Remove small onion pieces early if darkening too fast.
Serve immediately in 4 side portions, each with potatoes and softened onion.