In a medium saucepan, add the rinsed quinoa and water. Bring to a boil over medium-high heat. Once boiling, reduce the heat to low, cover, and let it simmer for about 15 minutes, or until all the water is absorbed. Remove from heat and let it sit, covered, for 5 minutes. Fluff with a fork and set aside.
While the quinoa is cooking, preheat your oven to 375°F (190°C). Gently clean the portobello mushroom caps with a damp cloth and remove the stems. Place them on a baking sheet, gill side up.
In a mixing bowl, combine the cooked quinoa, diced tomatoes, bread crumbs, chopped basil, and feta cheese. Drizzle with olive oil, balsamic vinegar, and sprinkle in the black pepper and crushed red pepper flakes. Season with kosher or sea salt to taste. Mix until everything is well combined.
Generously spoon the quinoa filling into each portobello cap, ensuring they are heaped well.
Drizzle a little olive oil over the stuffed mushrooms and place the baking sheet in the preheated oven. Bake for 25-30 minutes or until the mushrooms are tender and the tops are golden brown.
Remove the mushrooms from the oven, let them cool for a few minutes, and then serve warm. These quinoa stuffed portobello mushrooms are perfect on their own, but you can also pair them with a side salad for a complete meal!