Put the quinoa in a saucepan with the water and bouillon cube. Cover and bring to a simmer over medium-low heat. Cook for 15 to 17 minutes, or until all the water is absorbed. Fluff with a fork, remove from heat and let cool for 20 to 30 minutes (see note).

Preheat the oven to 400 degrees F. Line a large, rimmed baking pan with unbleached parchment paper. Put the mushrooms gill side up on the prepared pan. Drizzle 1⁄2 teaspoon olive oil and a dash of sea salt over the gills of each mushroom.

Put the cooled quinoa, sweet red pepper, walnuts, basil, cashews, garlic, capers, all-purpose seasoning, 1⁄8 teaspoon salt and pepper in a large bowl. Stir gently with a large spoon to combine. Put 1⁄3 heaping cup of the quinoa mixture into each mushroom, patting it down firmly as you go. Brush the top of the quinoa mixture on each mushroom with about 1⁄2 teaspoon olive oil, using a pastry brush or back of a small spoon.

Tent the mushrooms with foil. Bake for 50 minutes, or until the mushrooms are soft. Remove foil from the mushrooms and bake uncovered for 10 to 15 minutes, or until the top of the stuffing gets slightly golden. Put the pan on a wire rack and cool for 10 minutes before serving.