Prepare the filling: Place rinsed quinoa in a medium saucepan. Add the broth and bring to a rolling boil. Reduce heat to low, cover, and cook until the liquid is absorbed, about 15 minutes. Remove the pan from heat and let stand, covered, for 5 minutes. Fluff with a fork.
Preheat the oven to 350 degrees F. Heat a medium saucepan over medium heat. Add the oil and garlic and cook, stirring, until golden, about 1 minute. Add the tomatoes, salt, and pepper to taste, and cook, stirring, for 5 minutes to develop the flavors. Remove the pan from heat, add the cooked quinoa, Romano, spinach, basil and mozzarella.
For the peppers: Halve the peppers lengthwise and remove the core and seeds. Place the peppers cut-side-up in a baking dish. Fill each pepper with 1/2 cup of the filling. Top each with 1 tablespoon crushed tomatoes, 1/2 teaspoon Romano and 1 tablespoon mozzarella. Pour the broth into the bottom of the dish. Cover tightly with foil.
Bake until the peppers are soft, about 50 minutes. Remove them from the oven and let cool for 5 minutes before serving.