Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente, about 10-12 minutes. Drain and set aside.
While the pasta cooks, heat olive oil in a large skillet over medium heat.
Add the sliced kielbasa to the skillet and cook until browned on both sides, about 5 minutes. Remove kielbasa from the skillet and set aside.
In the same skillet, add the diced onion and red bell pepper. Sauté until the vegetables are softened, about 5 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the diced tomatoes, tomato sauce, dried Italian seasoning, and crushed red pepper flakes. Season with salt and black pepper to taste.
Simmer the sauce for 8-10 minutes, stirring occasionally, until it thickens slightly.
Return the browned kielbasa to the skillet and stir to combine with the sauce.
Add the cooked penne pasta to the skillet and toss everything together until the pasta is well coated with the sauce.
Sprinkle the shredded mozzarella cheese evenly over the pasta and cover the skillet with a lid. Cook for 2-3 minutes until the cheese melts.
Remove from heat and garnish with chopped fresh parsley before serving.