Preheat oven to 375°. Line a rimmed baking sheet with parchment paper.
Arrange mushrooms, stem side down, on prepared baking sheet.
Bake until liquid starts to release, 10 to 12 minutes. Let cool for 10 minutes. Transfer mushrooms to paper towels to absorb any extra liquid. Brush mushrooms with 2 teaspoons extra-virgin olive oil. Sprinkle with ½ teaspoon salt.
In a medium saucepan, combine quinoa and stock. Bring to a boil over medium heat. Reduce to medium-low heat and cover. Cook until quinoa is soft and translucent and stock is absorbed, 10 to 15 minutes. Transfer quinoa to a medium bowl. Add sweet potato, stirring gently until combined.
In a medium skillet, heat pure olive oil over medium heat. Add shallots and garlic, cooking until fragrant, approximately 1 minute. Add shallot mixture, parsley, oregano, pepper, lemon juice, and remaining ¼ teaspoon salt to quinoa mixture, stirring gently until combined. Let cool slightly or to room temperature.
Using a levered scoop slightly smaller than mushrooms, generously portion quinoa mixture into wells of each mushroom.
Garnish with oregano, if desired. Serve warm or at room temperature.
Recipe by TeaTime Magazine at https://teatimemagazine.com/quinoa-stuffed-mushrooms/