Ispanağı fazla sevmiyorum, ama marullu salatada biraz olunca sorun etmiyorum.

Two things are happening:

1. Accusative suffix is added: -ı

   • ıspanak → ıspanak-ı

2. Final k often softens to ğ before a vowel-initial suffix

   • ıspanak-ı → ıspanağı

So ıspanağı is the form of ıspanak with the accusative ending.

This kind of change is very common in Turkish:

• çocuk → çocuğu
• köpek → köpeği
• yemek → yemeği

Because ıspanağı is the direct object of sevmiyorum.

With verbs like sevmek, Turkish can use:

• a bare noun for a more general statement: Ispanak sevmiyorum
• an accusative-marked noun for something more specific, topical, or more clearly treated as the object: Ispanağı sevmiyorum

In real speech, both can occur, but ıspanağı here sounds very natural because the speaker is clearly talking about spinach as the item under discussion.