I would suggest not making this in the crockpot as the broccoli can get a little overpowering.
I saute 1/2 of a diced onion in 1 tablespoon of butter or canola oil until soft, seasoning with salt, pepper and a pinch of cayenne. Add the finely chopped broccoli and chicken stock along with one or two diced potatoes or leftover cooked rice. Cook it all together until the broccoli is very soft.
Add milk or evaporated milk to get the quantity you want, then start stirring in the cheese. You can use an immersion blender to make the entire thing smoother, or run it through a regular blender or foodmill. Serve immediately.
This soup freezes well if you use rice instead of potato for a thickener. I like brown jasmine rice.