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 for 5-6 minutes, stirring occasionally, until browned on both sides. Remove kielbasa from the skillet and set aside.
In the same skillet, add the diced onion and red bell pepper. Sauté for 5 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the diced tomatoes with their juice, tomato sauce, Italian seasoning, and crushed red pepper flakes. Bring to a simmer and cook for 5 minutes to blend flavors.
Return the browned kielbasa to the skillet and stir to combine. Cook for an additional 3 minutes.
Add the cooked pasta to the skillet and toss everything together until well combined. Season with salt and black pepper to taste.
Sprinkle the shredded mozzarella cheese over the pasta and cover the skillet. Let it sit for 2-3 minutes until the cheese melts.
Remove from heat and garnish with chopped fresh parsley before serving.