Add the rinsed quinoa and water (or vegetable broth) to a small pot. Bring to a boil over medium-high heat. Once boiling, reduce the heat to low, cover with a tight lid, and simmer for 12–15 minutes until the liquid is fully absorbed. Remove from heat and let it steam covered for 5 minutes before fluffing with a fork.

Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. Use a metal spoon to gently scrape out the dark gills from the underside of the cleaned mushroom caps. Lightly brush both sides of the caps with 2 tablespoons of olive oil and season the insides with salt and pepper. Place them gill-side up on the baking sheet and bake for 10 minutes. Remove from the oven and carefully dab away all pooled moisture from the center of the caps using a clean paper towel.

While the mushrooms are pre-baking, heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the minced garlic and sauté for 1 minute until fragrant. Toss in the chopped cherry tomatoes and chopped spinach. Cook for 2–3 minutes until the spinach is completely wilted and the tomatoes are tender. Remove the skillet from the heat.

Stir the cooked quinoa, chopped parsley, lemon zest, and lemon juice directly into the skillet with the sautéed vegetables. Season with salt and black pepper to taste. Gently fold in most of the crumbled feta cheese (if using), saving a tiny bit for the top.

Spoon the quinoa mixture generously into the pre-baked portobello caps, pressing it down slightly to stack it tightly. Top with any remaining feta cheese. Return to the oven and bake for 12–15 minutes until the mushrooms are fully tender and the tops are beautifully set and lightly golden.

Sprinkle with additional fresh parsley before serving hot.