Heat 1 tablespoon of butter and 1 tablespoon vegetable oil in a large pan over medium-high heat.
Sprinkle onion and peppers with 3/4 teaspoon seasoned salt and 1/4 teaspoon pepper. Place onions and peppers in heated pan and cook 6 to 8 minutes. Place in a bowl and set aside.
If necessary, add a little more oil to the pan. Place over medium-high heat. Sprinkle steak with 1/2 teaspoon seasoned salt, 1 teaspoon steak seasoning, and 1/4 teaspoon pepper.
Cook steak until seared, about 1 to 2 minutes per side. If you like your steak well done, cook a little longer. Place steak in bowl with onions and peppers.
Add 1 tablespoon of butter to pan and cook mushrooms until golden. Transfer to the bowl with steak, onions, and peppers.
Wipe the pan clean. For each quesadilla add about 1 teaspoon oil and 1 teaspoon butter to the pan. Place over medium heat.
For each quesadilla, place about 3 slices cheese on one half of tortilla. Top cheese with steak, onion, and pepper mixture, using desired amount. Layer 3 more slices of cheese on top of filling and fold tortilla in half.
Cook in heated pan until golden and crispy on each side. Repeat with remaining tortillas.