In fa-ẓalla al-imāʾ yamtalilna ḥuwārahā, ẓalla is a defective verb, al-imāʾ its subject, and the verbal clause yamtalilna ḥuwārahā serves as its predicate. The attached nūn marks a feminine plural verbal subject.
Ḥuwārahā is the direct object: the young camel being roasted. The verb construction presents a process in progress, so the verse describes the activity of preparing food, not merely a completed meal.