In a large skillet, melt butter over medium heat. Add onion; cook for 6 minutes. Add mushrooms and garlic; cook for 5 minutes, stirring frequently. Remove pan from heat, and spoon mushroom mixture into a bowl.

Season both sides of fillets with garlic salt and 1/4 teaspoon pepper.

In the same skillet, heat oil over medium heat. Cook steaks for 5 to 6 minutes on each side, or until desired degree of doneness.* Remove from pan and place on a baking sheet in a warm oven.

Add flour to drippings in skillet and cook, stirring constantly, for 2 minutes. Gradually add broth, whisking until smooth. Add wine, salt, and remaining 1/4 teaspoon pepper. Cook for 6 to 7 minutes, or until slightly thickened. Add mushroom mixture; cook for 1 minute, or until heated through.

Spoon sauce over steaks to serve.

The best way to ensure that meat reaches a safe internal temperature is to use a meat thermometer. This cuts out the guesswork and prevents overcooking or undercooking of meat. According to the USDA, beef tenderloin should be cooked to an internal temperature of at least 145 degrees F.