In a resealable bag, combine boneless, skinless chicken breasts with olive oil, minced garlic, dried oregano, seasoned salt, and ground black pepper. Shake vigorously to ensure the chicken is well-coated. Allow the chicken to marinate for at least 30 minutes or up to overnight in the refrigerator.

Heat a large skillet over medium heat. Once hot, add the marinated chicken breasts to the skillet, ensuring not to overcrowd the pan. Cook for approximately 5 minutes on each side or until the chicken is golden brown and reaches an internal temperature of 165°F. Remove the chicken from the skillet and set it aside to rest.

In the same skillet, melt a couple of tablespoons of butter over medium heat. Once melted, add sliced mushrooms and diced white onion, seasoning them with a pinch of salt. Sauté until the onions are translucent and the mushrooms are golden, about 5–7 minutes.

Return the cooked chicken to the skillet, nestling it among the sizzling mushrooms and onions. Generously sprinkle shredded Monterey Jack cheese over the chicken. Cover the skillet with a lid, allowing the cheese to melt for about 2–3 minutes.

Carefully place the smothered chicken onto plates, using a spatula to transfer it gently. Top each portion with the sautéed mushrooms and onions.