Hare Krishna, please accept my humble obeisances...
I am forced to offer food cooked by non-devotee most of the time... but it is not mixed with onions, garlic and mushrooms... I would make sure that tulasi leaves touch almost every part of the food... I would then offer it to the Lord... before leaving it for them to eat, I would touch His lotus, transcendental feet, and touch the food, just to double-purify the food... Will the Lord accept it? (I am lacking of time to cook nice food... and there is shortage of items to cook nice food for the Lord to eat)
Your humble servant,
Krishna Hari Dasa