Bring a large pot of water to a boil with 1 tablespoon of salt. Cook the pasta according to package instructions until al dente. Drain and set aside.

In a large mixing bowl, combine the sliced onion and minced garlic. Drizzle with olive oil and red wine vinegar, stirring to coat the onions and garlic. Add the cooked pasta to the bowl and toss well.

Add the halved grape tomatoes and baby spinach to the pasta mixture. Toss gently to combine. Season with salt and pepper to taste. Sprinkle with grated Parmesan cheese if desired. Serve immediately or refrigerate to chill.