A method of providing a heated food product comprising: sealing a pre-formed portion of food of essentially uniform thickness into an envelope of a film material, and cooking or heating the product by placing the resultant package between the plates of a clam-shell heater. The film used for the package is of a type which will form a rugged seal at freezer or ambient temperatures but which will vent at elevated temperatures and pressures with the result that as the food product is heated and steam pressure develops within the envelope, the envelope will self-vent. The envelope is removed when the food product is served, either immediately after heating or after an appropriate dwell time.