* "On a dish" is correct when the apple is resting on top of the dish. For example: "There was a red apple on a silver dish."
* "In a dish" is correct when the apple is contained within the dish, such as if it is sliced or part of a fruit salad. For example: "I put the apple slices in a bowl."
Therefore, both "on a dish" and "in a dish" can be grammatically correct, but the choice depends on the specific situation.