Prep the Oven: Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper to make cleanup easy.
Make the Garlic Parmesan Spread: In a small bowl, mix the softened butter with minced garlic, grated Parmesan cheese, and chopped parsley (if using).
Toast the Bread: Slice the baguette lengthwise. Generously spread the garlic Parmesan mixture over each half. Place the bread halves on the prepared baking sheet and toast in the oven for 6-8 minutes, or until golden and crisp. Remove from the oven and set aside.
Cook the Vegetables: In a large skillet, heat olive oil over medium-high heat. Add the sliced onions and bell peppers. Sauté for 5-6 minutes until soft and lightly caramelized.
Cook the Steak: Add the thinly sliced steak into the skillet with the veggies. Season with salt and pepper. Cook for about 3-5 minutes, stirring frequently, until the steak is cooked through.
Assemble the Sandwiches: Divide the steak and veggie mixture among the toasted bread halves. Top each half with slices of provolone cheese.
Melt the Cheese: Return the sandwiches to the oven and bake for another 5 minutes, or until the cheese is melted and bubbly.
Serve and Enjoy: Remove from the oven, slice, and serve hot. Optionally, sprinkle with fresh parsley or serve alongside chips or pickles for a touch of freshness!