Preheat oven to 350°F and grease a large baking dish or baking sheet with cooking spray or oil. Arrange the peppers in the baking dish and set aside.
Place the quinoa and vegetable broth in a medium sauce pot. Bring to a boil then stir, reduce to a simmer, cover and cook until liquid is fully absorbed, about 15 minutes. Remove the lid and fluff with a fork.
Meanwhile, add the olive oil to a large skillet over medium heat. Once hot, add the onion and sauté for 3 minutes until slightly softened.
Add the garlic and spices to the skillet, stir and cook until fragrant, about 1 minute.
Add the diced tomatoes and tomato paste, stir to combine and cook an additional 3-5 minutes until most of the liquid from the diced tomatoes is cooked off. Turn off the heat and set aside.
Place the cannellini beans, olives, lemon zest, lemon juice, feta and ¾ of the minced parsley in a large bowl.
Add the mixture from the skillet and the cooked quinoa. Mix until full combined. Taste and season with salt and pepper.
Spoon the mixture into the halved peppers and place on the prepared baking dish.
Cover the dish loosely with aluminum foil and bake for 25 to 35 minutes depending on how crisp you like the peppers.
Remove from the oven, top the chopped walnuts, reserved minced parsley and additional crumbled vegan feta, if desired.