ON breakfast or IN breakfast?

Both of “on” and “in” don’t work as prepositions to describe an addition to your breakfast as you have used them here.

The reason is that breakfast here is an entire thing – it is all the foods you are eating for that meal. You can’t talk about putting an additional item of food “on” or “in” your breakfast because then it would be your breakfast.

I can say I put a tablet “in” my dog’s breakfast because I am hiding something that isn’t really part of her breakfast inside it, so she eats it at the same time.

You can put a cloche “on” your breakfast or a garnish “on” your breakfast, since they aren’t thought of as part of your breakfast, just things sitting “on” it. You can even say “I like ketchup on my breakfast” because we can think of the sauce sitting “on” it as an extra, added after its preparation but not quite being incorporated into the breakfast itself.

To refer to additional items adding to your breakfast, or foods constituting the entirety of your breakfast, you need to use other prepositions like “with” or “for”.