Spray crock pot with non-stick cooking spray or use a slow cooker liner.
Cut meat into thin strips, place in slow cooker. (Tip: Put steak in freezer for 30 min to chill before slicing it! Cuts super thin and easy then!)
Add onion, stock, garlic salt, pepper and dressing mix.
Cover and cook on low 7-8 hours, or high for 3-4 hours.
The last ½ hour add bell peppers on top to soften.
Cook mushrooms in large frying pan with olive oil-season to taste.
Remove bell peppers and add to frying pan to finish sautéing them.
Remove both and keep warm.
Cut bread lengthwise.
Mix together 2 cloves garlic/3-4 tablespoons butter.
Butter the bread on both sides and place face down in fry pan to toast.
Place steak/onions from slow cooker onto the bread.
Add mushrooms and bell peppers on top.
Cover with 2 slices provolone on each sandwich.
Place on cooking sheet and broil them until cheese is nice and melty!