Preheat oven to 400 Degrees Fahrenheit.
In a small sauce pan, bring vegetable broth and quinoa to a boil. Cover and reduce to simmer, cooking for 13-15 minutes. Fluff with fork before allowing quinoa to cool.
While waiting for quinoa to cook, wash mushrooms well. Remove stems by gently pushing the stem to one side until you hear a quiet click (indicating the stem has detached); be careful to keep mushroom tops in-tact.
Finely dice mushroom stems and set aside.
In a medium frying pan, heat 2/3 of oil on medium. Sauté onions and mushroom stems (stirring often) until tender, 4-5 minutes. Remove from heat and allow to cool.
While waiting for mixture to cool, prepare a medium baking sheet with tinfoil and left-over oil. Evenly space the mushroom caps on baking sheet.
Stir egg whites, quinoa, walnuts, herbs and spices into cooled mushroom mixture.
Using a small spoon, scoop ~1 tsp of filling into the mushroom caps. Sprinkle each cap with ~1/3 tsp of cheese.
Bake until cheese is fully melted & begins to brown and mushrooms are tender, 12-14 minutes. Serve warm.