Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet or sauté pan over medium heat.
Add diced onion and minced garlic to the pan. Sauté for 2-3 minutes until the onions become translucent and fragrant.
Add diced bell pepper and chopped vegetables of your choice. Sauté for another 5 minutes until the vegetables are slightly tender.
Add the cooked quinoa, diced tomatoes, drained white beans, dried thyme, dried oregano, salt, and pepper to the skillet. Stir well to combine all the ingredients.
Pour in the vegetable broth and stir again to ensure the mixture is evenly moistened.
Add nutritional yeast to the skillet and mix it in thoroughly. The nutritional yeast will add a cheesy flavor to the casserole.
Transfer the mixture from the skillet to a greased baking dish, spreading it evenly.
Sprinkle breadcrumbs over the top of the casserole, creating a crispy and golden crust.
Cover the baking dish with foil and bake in the preheated oven for 25 minutes.
Remove the foil and continue baking for an additional 10-15 minutes until the casserole is bubbling and the breadcrumbs are lightly browned.
Remove the casserole from the oven and let it cool for a few minutes before serving.