Place flour, garlic salt, and 1/8 teaspoon black pepper in a ziploc bag. Add the beef and shake to coat. In a cast iron skillet over medium heat, melt 1 tablespoon of the butter. Add the beef, and cook stirring frequently.
Add in the sliced mushrooms and the remaining 1 tablespoon butter, stir until mushrooms are tender.
For the cognac sauce: In a medium sauce pan over medium high heat, melt 2 tablespoons butter. Whisk in the flour until a blonde roux has developed. Slowly pour in half of the beef broth, whisking to combine. Add in the browning sauce, the Worcestershire sauce, and remaining beef broth whisking to combine. Add in the black pepper, stir to combine. Continue stirring until slightly thickened. Add in the cognac during the last minute of cooking, stir to combine.
Serve the beef tips, mushrooms and cognac pepper sauce over your favourite mashed potatoes.