Add butter to a large Dutch oven over medium-high heat.
Once butter is melted, add onion, garlic, broccoli, ground mustard and cayenne and stir.
Add 1 cup of water and baking soda to the pot, stir and lower the heat to a simmer. Cover the Dutch oven with a lid and let broccoli cook until soft (about 20 minutes).
Once broccoli is soft, bring heat back up to medium-high and add remaining 2 cups of water and all the vegetable broth, stirring to combine.
Once mixture is gently bubbling, add spinach and stir. When spinach is wilted, add cheese to the mixture and continue to stir. Add salt & freshly ground black pepper to taste.
Using an immersion blender, blend until smooth and creamy. Fold in cooked quinoa and stir to combine.
Top with crackers or homemade croutons and additional cheese as desired.