1. Place onion in the bowl with the ultrablade knife. Mix at speed 12 for 10 seconds. Scrape down the sides and bottom of the bowl. Replace the ultrablade knife with the mixer blade. Add butter and launch the P1 simmer program.
2. Add flour, salt, and pepper. Mix at speed 2 at 270°F for 1 minute.
3. Add remaining ingredients, except half and half and parsley. Set P1 simmer program at 270°F for 12 minutes or until boiling. Add half and half and parsley and mix at speed 2 for 2 minutes.