Mustard is a standard sauce available in Papa's Cheeseria 2. The sauce is unlocked on Rank 48 with Brittany.
Mustard is a standard drizzle available in Papa's Fry Mia!. The drizzle is unlocked from the start of the game.
Mustard is a standard sauce available in Papa's Hamburgeria. The sauce is unlocked on Day 2 with Francis.
Mustard is a standard side that customers can order in Mama's Breakfasteria. It is unlocked on Rank 30.
Mustard is a standard topping sauce available in Papa's Poutineria. The topping sauce is available from the start of the game.