Preheat your oven to 375ºF.
In a casserole dish, combine the quinoa, marinara sauce, water, Italian seasoning, minced garlic, and half of the minced onion. Stir well until the quinoa is fully coated in the sauce.
Cover the casserole dish and bake for 30 minutes.
After 30 minutes, give the casserole a stir, ensuring the edges are wiped clean to prevent burning. Recover and bake for an additional 20-30 minutes until most of the liquid is absorbed, and the quinoa is tender.
Once cooked, uncover the casserole and sprinkle the shredded mozzarella cheese evenly over the top. Arrange the pepperoni slices on top of the cheese.
Bake the casserole uncovered for an additional 10 minutes until the cheese is melted and bubbly.
Remove from the oven and allow it to rest for 10 minutes before serving.
Garnish with dried oregano for an extra burst of flavor, and enjoy your hearty and nutritious meal!