Bring a large pot of water to a boil with 1 tablespoon of kosher salt. Cook the pasta according to package instructions until al dente. Drain and set aside.
While pasta is cooking, place the sliced red onion and minced garlic in a large mixing bowl. Add olive oil and red wine vinegar to cover the onions.
Add cooked pasta to the bowl and toss to combine. Then, add halved grape tomatoes and baby spinach. Toss again to mix well.
Season with salt and freshly ground black pepper. Taste and adjust seasoning as needed. Sprinkle with grated Parmesan cheese if desired. Serve immediately or chill for later.
Notes
This spinach pasta salad is best served fresh but can be refrigerated for up to a day. Add Parmesan just before serving.