Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente.

Drain the pasta in a colander, reserving 1/2 cup of the pasta water.

In a skillet, heat the olive oil over medium heat. Add the chopped red onion and sauté for 3-4 minutes until translucent.

Add the minced garlic and red pepper flakes, and sauté for an additional 1-2 minutes until fragrant.

Stir in the sun-dried tomatoes and chopped spinach. Cook until the spinach is wilted, about 2-3 minutes.

Add the cooked pasta to the skillet with the sautéed vegetables. Toss to combine, adding reserved pasta water as needed to create a light sauce.

Season with salt and black pepper to taste. If desired, stir in butter for added richness.

Serve hot, topped with grated Parmesan cheese.