In a large bowl add the olive oil, lemon juice, Dijon mustard, rosemary, garlic, salt, and pepper. Mix well.
Rinse the chicken with cold water, pat dry with paper towel, and add the chicken into the bowl with the seasonings. Add the quartered onions.
Toss well to coat all the chicken pieces evenly.
Open the lid, place the air fry tray inside of the basket. Select Roast and set the temperature to 375°F. Press Time and set for 30 Minutes. Press Start.
Once preheated and displays Add Food, add the chicken and onions, spreading out in an even layer. Lay the lemon slices overtop of the chicken and onions.
Once the cooking time has finished, transfer the chicken to a serving platter and serve immediately.