This is not meant to seem like a correction to Suzanne yet from experience with the TGIF recipe.... It used to be a just a Bechemel base (like you mentioned) with quartered artichokes, chopped spinach that was squeezed dry and shredded parmesan and jack cheese added together. Just no mayo. About 5 years ago it became more similar to my recipe with a change to sauteeing red peppers with onion and garlic and adding cream cheese, parmesan, artichokes,half and half, spices and then the spinach.
Personally I didn't think either we all that bad for a "chain". I favored the second more because of the depth of flavors and because they took away the chips and went to toasted ciabatta Another reason I favored this style was it resembled my recipe and since I liked to offer crostini, toasted pita and/or bagel chips for dipping instead of the standard tortilla chip. in a nutshell theirs became close to the one I've been using for the last 14 years with a measurable degree of success:
roux (to tighten)
S&P to taste
In variations of this I have added shrimp, crab meat, chopped lobster meat and even blackened or plain grilled chicken. You can try any combination and additional seasoning/herbs such as tarragon, rosemary, clove, etc, etc.