Learn Finnish: Eikö tämä ole isoäidin kirjoittama resepti? - Isn't this the recipe written by grandmother?

The agent participle is formed by taking the strong stem of a verb (here, kirjoitta- from kirjoittaa) and adding -ma or -mä. It acts like an adjective describing something that was done by a specific person or thing. Here, kirjoittama resepti means "the written recipe".

Why is the word for grandmother in the genitive form (isoäidin)?

When using the agent participle, the person or thing doing the action (the agent) must always be in the genitive case. Therefore, isoäidin kirjoittama resepti translates literally as "grandmother's written recipe", which means "the recipe written by grandmother".

Could I just use a relative clause with joka (which/who) instead?

Yes, you can! You could express the same idea by saying Eikö tämä ole resepti, jonka isoäiti kirjoitti? (Isn't this the recipe that grandmother wrote?). The agent participle structure is simply a more compact way to express the same thing.

Why do we use ole here instead of on?

This is because of the word eikö. Eikö is made up of the negative verb ei plus the question particle -kö. In a negative sentence, the main verb drops its personal ending and takes the "connegative" form, which for olla is ole.