Melt butter in heavy large saucepan over medium heat. Add onion and saute until tender, about 10 minutes. Add garlic, epazote and bay leaf. Stir 1 minute. Add quinoa and stir just until coated. Add stock. Bring to boil. Reduce heat and simmer until quinoa is tender, about 10 minutes. Drain. Discard epazote and bay leaf.
Transfer pilaf to large bowl. Season to taste with salt and pepper. Garnish with parsley or cilantro.