Reserve a few flowerets of broccoli for garnish. Cook broccoli in boiling, salted water until just done, firm and very green. Place the broccoli, broken into small pieces, stems included, into a pan with the butter and minced onion. Cover and stream for 10 minutes. Stir once or twice, do not brown.
Add the rice and stock, bring to boil, lower the flame and simmer 30 minutes. Strain through a fine sieve, forcing the vegetables through. Return to kettle, season, add the cream and butter. Heat again and serve with the reserved flowerets as garnish.
To make this soup in a blender, cook as directed and then put the mixture, 2 cups at a time, through the blender, putting some of the solids in with each portion. Blend until a smooth puree is obtained. Return to kettle, add cream and butter, season and heat.
Serve with a little salted whipped cream, if desired, and the broccoli flowerets on top.