Bring a large pot of salted 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 1 tablespoon minced garlic and sauté for another 60 seconds.
Add in 1/4 cup red wine vinegar and 1/2 cup balsamic vinegart will sizzle like crazy and reduce rather quickly.
By this time your pasta should be done and drained. Turn off the heat to the skillet and add the cooked pasta, 1 pint cherry or grape tomatoes and 3 cups baby spinach. Stir to coat the noodles and continue stirring until the spinach begins to wilt, about 1 to 2 minutes. Taste, and season with salt and pepper if necessary.
Sprinkle 1/2 cup crumbled feta cheese on top and serve.