All 6 of the ingredients are key items you get by completing tasks for different characters.
Fairy dust from Ophelia the fairy
The perfect bagel from Sweet Cravings
Wagyu beef from the waitress at the all-you-can-eat
High quality greens from Diane
Flavor enhancers from Cici's quest line.
Homemade condiments from the hotdog eating contest.
Remember that these are all key items, so they'll show up on your inventory screen, not in your ingredients list.
The last one you get is for the hotdog eating contest. You need to get to the highest weight level in the game to beat the last contest, so the sandwich is very much end game content. As for the rest, you need the vials for the bagel and the wagyu beef and the gold dabloon for the fairy, both are bought at the curios shop, and both the flavor enhancers and the High quality greens are locked behind Tiffany so you need to unlock the lab for them(Also one of Cici's endings result in Tiffany going away but don't worry, that doesn't stop you from doing Diane's stuff).
