Heat 2tbsp butter in a medium saucepan, add onion, garlic, chopped broccoli, dry mustard powder, cayenne and salt. Cook on medium-high flame, stirring constantly, until it starts to smell fragrant.
Add 1cup water and baking soda. Bring the mixture to a boil; lower the heat to simmer, cover and cook for 15~20 minutes until broccoli is soft.
Add 2cups of water and the bouillon cube (or just add 2 cups of vegetable broth). Bring the mixture to a simmer and add the spinach. Cook until the greens wilt, about 1~2 minutes.
Turn off the heat and stir in the cheeses. Blend the soup either using immersion blender or regular blender until smooth. Return the soup to the pan and bring to a simmer. Serve immediately.