To eat, right click on Grilled Fish in your inventory and then Consume.
Simple and portable, Grilled Fish provides a small temporary (10 minute) bonus of +1 Endurance when eaten.
As meals do not stack, the bonuses from eating grilled food will replace any meal-related bonuses and penalties that were already in effect (from a Kitchen meal).
A grilled vegetable or fish now stacks with another type (i.e. eating a grilled onion and then eating a grilled cabbage will give you +2 speed and +2 endurance). However, eating two vegetables of the same type does not stack (i.e. eating 2 grilled onions gives +2 endurance, NOT +4)
See the Grilled Food page for other grilled info.