I have no idea how Brussels Airlines manage their meal ordering system, but even when specifying your request beforehand I think there is a big distinction to be made between preferences (such as a vegetarian meal) and medical necessities (such as a diabetic meal).
As someone who requires a diabetic meal I am rarely let down by airlines — though it does happen and some cabin crew are very careless about snacks, even though they know that the patient has had a diabetic meal — but I always carry suitable protein bars when travelling, so I won’t be stuck for suitable food.
Actually, when ordering a special meal you may find that you get better attention as a result as alert cabin crew will check at boarding time who you are and that you are occupying the seat specified in their manifest. The often-criticised (though not by me) China Southern are particularly good in that respect. KLM are very good at providing alternative sugar-free snack items on request.
You have no recourse for compensation other than perhaps to call and ask for a few miles to be credited to your frequent flier account. Brussels Airline is a reputable airline, they will likely agree to do that. They do not like negative publicity.
Meals on most airlines are not part of the contract, they are a courtesy offered to you and have zero monetary value when not provided. You could not find out anything about compensation for meals because it does not exist.
It was an unfortunate situation however you are not going to get any traction on that. Even in the USA which is the hotbed of litigation where we sue for anything and everything, you would not get anything out of the courts.
CONCLUSION
Call the frequent flyer number and complain enough and you will get some miles credited to your account otherwise fuhgeddaboudit.
Credit:stackoverflow.com‘