Ultimate Steak Sandwich

Ingredients

• 1 lb round marinating steak
• 1/2 tsp salt
• 1/2 tsp ground black pepper
• 1/2 tsp garlic powder
• 1 tbsp unsalted butter
• 1 tbsp olive oil
• 1 small chopped onion
• 3 cloves chopped garlic
• 1/2 small red bell pepper, sliced
• 2 medium sub buns
• 2 tbsp dijon mustard
• 2 tsp mayo
• 1/4 cup shredded mozzarella