Slice steak thinly against the grain, and chop onion, bell pepper, and mushrooms.

In a large skillet, heat olive oil over medium heat, cook onions and bell pepper for 5-6 minutes.

Add mushrooms and garlic powder, cooking for another 3-4 minutes until fragrant.

Push veggies aside in the skillet, add steak, sear for 2-3 minutes each side. Season with salt, pepper.

Combine cooked vegetables with steak and mix well, allowing flavors to meld for an additional minute.

In a small saucepan, melt butter, whisk in flour, then add milk until thickened. Stir in cheese until smooth.

Serve beef and vegetables in bowls, drizzling warm cheese sauce over the top and garnishing with parsley.