Rinse the quinoa under cold water. Place it in a medium saucepan with the vegetable broth and bring to a boil. Reduce heat, cover, and simmer for 15 minutes until the liquid is absorbed.
While the quinoa cooks, cut the tops off the bell peppers and scoop out the seeds and membranes. Use a small knife to carve simple jack-o'-lantern faces into the sides of each pepper.
In a skillet over medium heat, cook the onion and garlic for 3 minutes until softened. Stir in the drained tomatoes, spinach, oregano, salt, and black pepper. Cook for another 2 minutes.
Fluff the cooked quinoa and mix it into the skillet with the vegetable mixture. Remove from heat and stir in half the cheddar cheese.
Spoon the filling evenly into the carved peppers. Place the peppers in a baking dish and sprinkle the remaining cheese on top.
Bake at 375°F for 25 minutes until the peppers are tender and the cheese is melted.