Leftovers are best stored separate. Leave assembly for when you’re ready to enjoy your burgers! The arugula, tomatoes, arugula, and buns are best fresh. The patties will be good for 3-4 days in an airtight container in the fridge.
To reheat, you can pop the patties in the microwave at 30 second intervals until they’re heated through. You can also place them in a skillet, covered, over medium heat until they’re warm.
For the onions, you can honestly enjoy these on your burger cold! If you prefer them hot, you can definitely reheat them in the microwave or in a skillet. I would suggest lining your plate with paper towel to microwave them. This will keep them as crisp as possible.