Fill a medium sized pot with water and fit with a steamer attachment. Bring the water to a boil and steam the broccoli florets until crisp-tender (about 3-4 minutes). Remove them from heat.
Blend all of the sauce ingredients together in a high speed blender or a food processor until they're totally smooth.
Preheat your oven to 350F. Heat the olive oil in a large skillet over medium heat. Add the onions and celery, and sauté the vegetables until the onion is clear and soft (about 7 minutes). Add the quinoa and the broccoli to the skillet. Add the sauce and stir everything to heat it through.
Transfer all ingredients to an oiled 7x11 or 8x12 baking dish. Top with breadcrumbs, if using. Bake for 30-35 minutes, or until the casserole is bubbling gently and the breadcrumbs are golden. Allow to cool for 15-20 minutes before cutting into squares and serving.