I'd kinda be thinking a fam style Italian thing if that agrees w/ everyone. Passed platters of pasta in red sauce, pasta in alfredo sauce, some italian sausage, sauteed spinach, chicken franchese, garlic bread - but then, you can sprinkle in some stuffed squid, or braciole, or anything else that you and others in the group might find a little more interesting, but the picky ppl can pass on, and still be extremely well fed&happy.
I'd also suggest tht for 15 - 20 ppl, you contact the joint before hand and work out a fam style menu, but that's me. I find it simply makes everything run smoother for the kitchen staff which translates into a better meal. Maybe something like:
Family style tossed salad with house italian dressing with bread / butter service
A few platters of a rigatoni with marinara, a few platters of fettucine alfredo.
A few platters of chicken francese, a platter of sausage&peppers, a fish dish, and a platter of braciole with some garlic bread.
See if they'll do a cake service if you bring one in? Maybe they'll have some gelati (is that the plural?) for you guys to pick from? Perhaps they can tray up some dessert samplers for you?
I would imagine that ANY Italian place that is open on Sunday would be thrilled to set this up for you for a post lunchy meal.
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