Melt butter in a saucepan over medium heat. Add the quinoa, and toast, stirring occasionally, until lightly browned, about 5 minutes. Stir in broth, and bring to a boil. Reduce to a simmer, cover, and cook for 15 minutes, or until quinoa is tender. Meanwhile, sauté mushrooms, onions, and garlic in olive oil until softened.
In a bowl, toss quinoa together with sauteed vegetables, thyme, and salt. Sprinkle with lemon juice, and serve.