What responsibility, if any, does the customer bear in avoiding harm to himself?
The onions in question are a burger topping, and are readily discoverable if the customer checks their order. I think that the customer with the special requirement can be reasonably expected to verify their order meets their needs before incurring harm.
I believe he's already suing Sonic for the same issue. He knew (or should have known) this was a mistake that restaurants can potentially make, yet he apparently made no effort of his own to mitigate the risk by checking his food before eating.
I would argue that it is "reckless" for the customer to blindly trust the worker fulfilled the special instructions, and that this "recklessness" is the primary cause of the harm incurred.
I would say that the restaurant's liability here is the cost of the "defective" burger.
Its only "basic needs". Paraplegics are still human. you dont habe your fingers anymore, but you've still got two extra arms, two extra legs, a spare lung and kidney, and a bunch of other stuff that you don't technically "need" as a human.