Bring a large pot of salted water to a boil and cook the penne until just al dente, about 1 minute less than package directions. Drain and set aside.
In a sauté pan, heat the olive oil over medium heat until it shimmers and smells fragrant. Add the chopped onion and cook, stirring occasionally, until translucent and soft, about 5 minutes.
Add the minced garlic to the pan and cook for another minute until fragrant, being careful not to burn it. Then, toss in your mixed vegetables and sauté until tender, about 5-7 minutes.
In a mixing bowl, combine the vegan cheese, nutritional yeast, and plant-based milk. Whisk until smooth and creamy—this will be your cheesy sauce.
Fold the cooked pasta into the sautéed vegetables and pour the cheese sauce over everything. Mix gently to coat all the ingredients evenly, creating a creamy, cheesy mixture.
Transfer the mixture into a deep baking dish, spreading it out evenly. Top with extra vegan cheese or breadcrumbs if you like a crispy crust.
Bake in a preheated oven at 180°C (350°F) for 25-30 minutes, until bubbling around the edges and the top is golden brown. The aroma of melted cheese will fill your kitchen.
Remove from the oven and let it rest for 5 minutes. This helps the sauce set slightly and makes serving easier.
Garnish with freshly chopped herbs for a burst of color and freshness. Serve hot and enjoy the cheesy, crispy edges and tender pasta inside.