Cook 1/4 cup quinoa as the label directs, then spread on a baking sheet to cool.
Preheat the oven to 425 degrees F. Remove the stems from 24 large cremini mushrooms.
Wipe the caps clean with a damp paper towel, then toss with 2 tablespoons olive oil and season with salt. Arrange on a baking sheet.
Combine the cooled quinoa, 1/2 cup each chopped scallions and chopped toasted walnuts, 1/4 cup chopped dill and/or parsley, 1/3 cup crumbled feta and 2 tablespoons chopped sun-dried tomatoes in a bowl. Drizzle with olive oil, season with salt and toss. Divide among the mushroom caps.
Bake until the mushrooms are tender and the filling is golden, 15 to 17 minutes. Let cool 5 minutes, then remove from the baking sheet.