Cut meat into thin strips by slicing diagonally across the grain. Set aside.
Cook pasta as directed. Drain and set aside.
Whisk together milk and flour in a small bowl or cup. Set aside.
While pasta is cooking heat oil in a large skillet over medium heat. Add mushrooms, onions, garlic, and onion powder/granules. Cook until mushrooms and onions start to soften.
Add steak and stir frequently, cooking until mushrooms are tender and steak is no longer pink, about 7 minutes.
Add the milk mixture to the skillet. Bring to a simmer. Stir gently until sauce thickens, about 1 to 2 minutes.
Fold in sour cream and peas and return to a simmer. Season to taste with salt and pepper.
Serve in bowls or on a plate over pasta. Sprinkle with parmesan cheese.