Peel the onions and trim the tops. Using a sharp knife, make several deep cuts from the top toward the base of each onion, stopping about ½ inch from the bottom so the onion stays intact. Gently spread the layers apart to create a blossom. Carefully remove a small portion of the center and finely dice it.
In a skillet, sauté the diced onion, mushrooms and garlic with a few tablespoons of water (if needed to prevent sticking) until the onions begin to soften. Stir in the spinach and cook just until wilted. Add the cooked quinoa, Italian seasoning, salt and pepper, stirring until everything is well combined.
Carefully spoon the filling into the center of each onion, gently tucking a little between the opened layers.
Arrange the onions in a baking dish. Cover with parchment paper, then foil and bake at 375°F for 35–40 minutes. Remove the foil and bake for an additional 5–10 minutes or until the onions are tender and lightly browned.
Drizzle with one of my signature sauces just before serving for added flavor and garnish with fresh parsley, if desired.