4 – 8 slices sourdough bread (I used 2 slices per sandwich, but you can make this open face and just use one thicker slice per sandwich)
4 slices sharp cheddar cheese
In a large skillet using the olive oil over medium heat cook onion and mushrooms seasoned with ¼ teaspoon seasoned salt until onion is translucent; set aside and keep warm.
Pound chicken down to a ¼ inch and sprinkle remaining seasoned salt on both sides.
Prepare grill for medium heat and lightly oil cooking grates. Place chicken on the grill, using 2 tablespoons of mustard mayonnaise; brush sauce on chicken, turning a couple of times to coat. Grill covered for 10 – 15 minutes (watch closely chicken will cook quickly at ¼ inch thickness.)
Divide and spoon the onion-mushroom mixture over the top of the chicken, top with cheese, close grill, and cook until cheese is melted; lightly toast bread on the grill.
Spread remaining mustard sauce over 1 side of 4 pieces of bread and top bread with the chicken to form the sandwich; top with the other piece of bread if using.