Maybe asking a little much, but each piece of food could be assigned a 'nutrition value' and to fully benefit from a Long Rest you need to eat/have available food equal to your constitution in value.
That's probably too much work to create for an already overworked design staff, but it offers solutions to both the 'problem' of magic-healing food and also the problems of Long-Rest-spamming. If you don't have the food for a meal, you only count as having had a short rest instead.
