Simmer balsamic vinegar for 10 minutes to create glaze while preparing mise en place

In a large pot of boiling salted water, cook pasta until al dente. Reserve 1/2 cup pasta water before draining. Return pasta to pot.

Coat both sides of steak with oil and season generously with salt and pepper. In a large skillet over medium-high heat, cook steak to desired doneness, 4 minutes per side for medium-rare. Transfer to a plate to let rest for 10 minutes. Thinly slice steak.

In skillet, melt butter over medium heat. Add garlic and cook until softened and fragrant, 1 to 2 minutes. Whisk in flour and cook 1 minute more, then slowly add milk, whisking to break up lumps, and simmer until thickened, 5 minutes. Add Parmesan and parsley and season with salt and pepper. Add tomatoes and cook until bursting, 2 to 3 minutes.

Add cooked pasta and 1/4 cup reserved pasta water to skillet and toss to combine, adding more pasta water as needed. Add spinach and toss until wilted.

Serve to plates, then top with sliced steak and drizzle with balsamic glaze.