Translation:I ate the porridge right away, and the orange an hour later.
i think your suggestion highlights why "i ate the porridge right away and the orange in an hour" is not only awkward, but probably wrong! If "i ate the porridge right away and the orange one hour later", then i would understand this as both having taken place in the past (e.g. porridge 2 hours ago and apple 1 hour ago). But "in an hour" is in my understanding used for something which will happen in the future (e.g. porridge 6 minutes ago, apple in one hour). But the Russian sentence has only one verb, and it is in the past tense. And maybe worth noting the verb isn't only in past tense, but perfective as well.
It's a bit semantically awkward. Usually "in an hour" is either used with the future (as discussed) to specify a time: "I will help you in an hour", or to indicate how long the activity will take: "He did it in an hour". If you mix these you can create ambiguity, such as: "I can do it in an hour". What's meant is sometimes clear from context, but not always.
Your sentence would be better if you said "...after an hour". It seems to me через can be translated with either "in" or "after", but in this case the latter is better.