In a large saucepan, bring water to a boil. Cook 1 pound penne pasta according to package directions.
Meanwhile, heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add in sliced 1 medium red onion and sliced 8 ounces white or cremini mushrooms. Season with salt and pepper and sauté for about 5 minutes.
Add in the 3 cloves minced garlic and sauté for another minute.
Add in the 1/4 cup red wine vinegar and 1/2 cup balsamic vinegar. It'll sizzle like crazy and reduce rather quickly.
By this time your pasta should be done and drained. Turn off the heat and add in the cooked pasta, 1 pint cherry or grape tomatoes and 3 cups baby spinach. Stir to coat the noodles and continue until the spinach begins to wilt. It should only take a minute or two. Taste, and season with more salt and pepper if necessary.
Sprinkle 1/2 cup crumbled feta cheese on top and serve.