etiquette - go to her and say, politely, that she appears to have thrown your lunch in the bin, and you had to buy replacement at cost of xxx, and could she please refund you by close of day.
Make a lovely passive aggressive notice for fridge door:
This fridge is for the use of ALL staff.
As it is used for lunches, the shelves should not also be used for raw meat, and this is a health and hygiene issue.
Please do not throw away anything in the fridge that isn't yours.