You can't really easily limit current draw from the car, but the cooker may draw less current when the environment is relatively warm.(room temperature) The rated current may be true for a lower temperature, and actual current draw may differ in reality, ie. it can be lower. You can try to run it with the 7.5A fuse and see if it doesn't blow. Simple to try and the worst thing you can do is blow a fuse, which is easily replaceable.
You found a 7.5A fuse in the fusebox, but maybe it's put there by a previous owner when that was the only fuse he had lying around. In other words, chances are the original fuse could be rater higher. Lots of cars use 15A fuses in that place. If you car also originally does so, it's an easy fix to put in a bigger fuse.
Otherwise, you need to connect a separate wire from the outlet to the fusebox and connect it to a main 12V wire. You can either use one that always gets power from the battery, or only when the key is in 1st or 2nd position.