What Happened to the Sweet Bun?

You want to say that you began eating a sweet bun at noon and finished it now.

A clear sentence is: “I started eating a sweet bun at noon, and I have just finished it.”

Use “started” because the beginning was in the past.

“Have just finished” means that you finished a moment ago.

This sentence uses the past simple and the present perfect.

It does not use the past continuous.

The past continuous form is “was eating”.

Say, “I was eating a sweet bun at noon,” if you were eating it at that time.

This sentence does not tell us when you started eating.

You can also say, “I have just finished eating my sweet bun.”

The present perfect can connect a past action to now.

Use “since noon” when an action started at noon and continues now.

For example, say, “I have been eating since noon.”

If you have finished, say “I have just finished” instead.

So “I started eating at noon, and I have just finished it” is a natural choice.