Rinse 1 cup of quinoa under cold water to remove its natural coating. In a medium pot, combine the quinoa with 2 cups of water or low-sodium vegetable broth. Bring to a boil, then reduce to a simmer, cover, and cook for about 15 minutes or until the quinoa is tender and the water is absorbed. Fluff with a fork.

Preheat your oven to 375°F (190°C). Remove the stems from the portobello mushrooms and gently scrape out the gills using a spoon to create space for the quinoa stuffing. Lightly brush the mushroom caps with olive oil and season with a pinch of salt and pepper. Place the mushrooms on a baking sheet, gill-side up.

Heat 1 tablespoon of olive oil in a pan over medium heat. Add the diced onion and garlic, sautéing for 2-3 minutes until softened and fragrant.

Add the cooked quinoa to the pan with the onions and garlic. Stir in the oregano, black pepper, and any additional salt to taste. Cook for another 2-3 minutes, allowing the flavors to combine. Optionally, squeeze in the lemon juice for added brightness.

Spoon the quinoa mixture into each mushroom cap, pressing gently to pack the filling.

Place the stuffed mushrooms in the preheated oven and bake for 20-25 minutes, or until the mushrooms are tender and the quinoa is slightly crispy on top.

Garnish with fresh parsley if desired, and serve the stuffed mushrooms hot.