Heat olive oil in a skillet over medium heat.
Brown the sausage with the onion, breaking it up as it cooks.
Stir in garlic for 30 seconds, then drain excess fat.
Combine marinara and broth in a bowl; stir until evenly blended.
Mix ricotta, 1 cup mozzarella, Parmesan, Italian seasoning, salt, pepper, and red pepper flakes.
Grease the slow cooker crock lightly with a bit of oil.
Spread a thin layer of sauce in the bottom of the slow cooker.
Add half the pasta, half the sausage, and spoonfuls of half the ricotta mixture.
Pour in more sauce to cover, then repeat the layers.
Finish with remaining sauce and the reserved mozzarella on top.
Cover and cook on LOW 3 to 3 1/2 hours, until Penne is tender.
Let rest 10 minutes, then sprinkle with basil or parsley and serve.