Learn Farsi: قبل از مهمانی، باید همه‌ی مواد غلیظ را خوب هم بزنید. - Before the party, you must stir all the thick ingredients well.

Because it follows باید (must). In Farsi, modal verbs like باید require the main verb to be in the subjunctive mood, which usually takes the بـ (be-) prefix. For the compound verb هم زدن (to stir), the prefix goes on the verbal part, creating هم بزنید.

What is the singular form of مواد (ingredients)?

The singular is ماده (mâdde), meaning "substance", "matter", or "ingredient". مواد is an Arabic "broken plural" (an irregular plural form). You will see it used very often in recipes to mean "ingredients".

Why does the object marker را come after غلیظ instead of right after مواد?

را marks the entire noun phrase as the specific direct object. Because the words are linked by the ezâfe into one complete phrase—همه‌ی مواد غلیظ (all the thick ingredients)—the را must go at the very end of that chain.

Does هم in هم بزنید mean "also" here?

No, in this sentence it is the first half of the compound verb هم زدن, which means "to stir" or "to mix". While هم on its own often means "also" or "together", here it functions purely as the non-verbal part of the compound verb.