In a large pot of salted boiling water, cook pasta according to package directions until al dente. Drain and set aside.
Coat both sides of steak with oil and season generously with salt and pepper. In a large skillet over medium-high heat, cook steak until your desired doneness, 4 minutes per side for medium rare. Transfer to a plate to let rest, 10 minutes, and then thinly slice.
Meanwhile, make alfredo sauce: Add butter to skillet and let melt. Add garlic and cook until fragrant, 1 minute. Whisk in flour and cook 1 minute more. Add milk and simmer until thickened, 5 minutes. Add parsley, Parmesan, salt, and pepper. Add tomatoes and cook down 2 minutes.
Add cooked pasta to sauce and toss until coated. Then, add spinach and toss until wilted.
Top with sliced steak and drizzle with balsamic vinegar.
