Bring a large pot of generously salted water to a rolling boil. Add the penne pasta and cook according to package directions until al dente, about 8-10 minutes. Before draining, reserve 1/2 cup of the starchy pasta water.

While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet or Dutch oven over medium-high heat. Add the sliced chicken sausage and cook for 5-7 minutes, stirring occasionally, until lightly browned and heated through. Remove the sausage from the skillet and set aside, leaving any rendered fat in the pan.

Reduce the heat to medium. Add the remaining 1 tablespoon of olive oil to the skillet, along with the chopped onion. Sauté for 5-7 minutes, until the onion softens and becomes translucent.

Stir in the minced garlic and red pepper flakes. Cook for another minute until fragrant, being careful not to burn the garlic.

Pour in the crushed tomatoes and chicken broth. Add the chopped sun-dried tomatoes (if using) and dried oregano. Season with 1/2 teaspoon of salt and 1/4 teaspoon of black pepper. Bring the sauce to a gentle simmer, then reduce heat to low, cover, and let it cook for 10-15 minutes to allow the flavors to meld.

Uncover the sauce and return the cooked chicken sausage to the skillet. Add the fresh spinach, stirring it into the sauce until it wilts, about 2-3 minutes.

Drain the cooked pasta and add it directly to the skillet with the sauce. Toss well to coat the pasta evenly. If the sauce seems too thick, add a tablespoon or two of the reserved pasta water until it reaches your desired consistency.

Serve immediately, garnished with fresh chopped basil and grated Parmesan cheese.