Melt butter in a large soup pot and add diced onions. Cook over medium heat for 3 minutes, when they begin to get translucent.
Add broccoli florets and broth and stir. Reduce heat to medium-low and cook for 7 minutes.
While soup is simmering, make your cashew cream by blending 1/2 cup raw cashews and 1 cup water for 1 minute. Then add the vegan cheese if using, along with the nutritional yeast and garlic powder and blend for another 30 seconds to fully incorporate.
Add the soup to the blender, reserving a few small broccoli florets to add to the soup bowls when serving. Blend soup with cashew cream for a minute and return to the pot.
Cook on medium-low for 2 more minutes and taste, adding more nutritional yeast or garlic if necessary.
Serve with oyster crackers or bread, topped with reserved broccoli florets and cheese shreds if desired.