Ιf you are going to use πάνω σε there is a high chance you will follow it up with έναν/μια/ένα because you are not talking about something really spesific, like on the example above the egg is on a (random) plate. If a mother were to make an egg for her kid she would probably say: "Το αυγό είναι πάνω στο πιάτο" meaning 'The egg is on the plate', hopefully this answered your question
Πάνω is actually an adverb in greek. Sometimes followed by a preposition, sometimes not. I can't think of any general rule on how πάνω (with or without a preposition) is translated in english, but here are some examples anyway.
Πάνω στο τραπέζι - On the table.
Πάνω από το κεφάλι σου - Above your head.
Πάνω από μία ώρα - Over an hour.
Σήκω πάνω - Get up.