Heat the olive oil over medium-high heat in a large saucepan or non-reactive skillet. After 90 seconds, when the oil is hot, add the vegan Italian sausage and diced onion, cooking for 6-7 minutes until the onion is soft and the sausage is slightly browned.
Stir marinara sauce into the skillet with the sausage and onion. Let it simmer for a couple of minutes. Set aside.
In a separate saucepan, melt vegan butter over medium heat. Add sliced garlic and sauté for a couple of minutes until fragrant, but not browned.
Reduce the heat to low and add vegan mozzarella shreds to the pan with garlic, stirring until the cheese has melted. Slowly mix in unsweetened plant-based milk until the mixture becomes smooth and creamy. Remove from heat and set aside.
Preheat your oven to 400°F (200°C).
Cook dried ziti or penne pasta according to package instructions until al dente, then drain.
Spread a few spoonfuls of sauce at the bottom of a 13-inch baking dish. Layer some pasta over this, followed by more sauce, about half of the cheese sauce, and a layer of ricotta. Repeat these layers until the dish is full and you've used all the ingredients.
Cover the pan with aluminum foil and bake the ziti for 25 minutes at 400°F (200°C). Remove the foil and continue to bake for an additional 6 minutes.
To serve, let it cool briefly before adding a garnish of fresh parsley, fresh basil, and a generous dose of grated vegan parmesan cheese.