We'll take them at their word that they've truly narrowed the variables to tuna and house sauce (i.e. they've eaten a meal consisting of only tuna and house sauce and gotten sick, at least one of which has always been the underlying cause, but everything else they've eaten has been properly eliminated, and there are no ways outside of the food truck they could've gotten sick), and thus the only logical options are T, HS, or T+HS. The premise of the joke already relies on completely unrealistic simplifying assumptions, so we can too.
Edit: We will not do this because it's logically impossible based on the described experiment thus far. I'm an utter dipshit.