Cook pasta shells according to package directions and set aside on wax paper to cool.
Cut beef into very thin strips.
Slice the red bell pepper, green bell pepper, onion, and mushrooms very thin.
Cook the meat in a skillet with 1 Tablespoon butter, 1 Tablespoon oil and garlic. Cook over medium heat until no longer pink.
When done cooking, put meat on a plate and set aside.
Add the other Tablespoon of butter and vegetable oil back into the skillet and cook the veggies until soft.
When the veggies are tender, add in the beef broth and the meat back in with the veggies.
Season with garlic salt, Italian seasoning, and salt and pepper.
In a small bowl mix together 1 Tablespoon of the cornstarch and water. Pour over the mixture to thicken up so it can be spooned into the shells.
Fill each pasta shell with a spoonful of meat and veggie mixture.
Place in a baking dish that has been lightly sprayed with nonstick cooking spray.
Slice 3 slices of the provolone cheese into small pieces and place inside each filled shell. Tuck the cheese inside the shell so it doesn't hang over the sides.
Prepare the cheese sauce by heating the milk, beef broth and 1 Tablespoon cornstarch in a saucepan over medium heat.
Slow add in the remaining provolone cheese and stir until completely melted.
Pour 1/2 of the cheese mixture around the shells and in the bottom of the pan.
Cook for 10 minutes at 350 degrees.
Remove from oven and spoon the remaining cheese sauce over each shell.