Fit a Cuisinart Food Processor with the metal chopping blade. With the machine running, drop the Parmesan through the feed tube to chop. Remove and reserve. Chop the garlic in the same fashion, with the machine running. Add the onion to the work bowl and pulse about 10 times to chop.
Place a Cuisinart 3.5 quart sauté over medium heat and add the butter. When butter melts, add the chopped onion and garlic to the pan. Add a pinch of salt and pepper to the onions and sweat over medium to medium-low heat until onions are completely soft, about 15 minutes.
While onions are cooking, remove any moisture out of the spinach by wrapping it in a tea towel and wringing it over the sink.
Once onions are cooked, place scallion in the work bowl of the food processor. Pulse about 12 times to chop. Add the cream cheese and pulse until processed. Pulse in the sour cream and mayonnaise. Add the reserved Parmesan, onions, spinach, basil, and garlic powder. Pulse all ingredients together until all is mixed in and homogenous. Taste and adjust seasoning accordingly
Remove and transfer to a small bowl. Serve immediately or wrap and place in the refrigerator. Flavors will bloom and come together after about one hour.