Melt the butter in a large Dutch oven over medium heat. Add the broccoli stems, onions, dry mustard and some salt and pepper and saute until the onions are translucent, about 5 minutes. Deglaze with the sherry and then add the broccoli florets. Add the stock and simmer for 5 minutes.
Carefully ladle everything into a blender along with the fresh spinach, Cheddar, cream and hot sauce and process until smooth, about 1 minute. Adjust with salt and pepper if necessary and adjust the thickness with more cream if necessary. Top with croutons and serve.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.