Learn Croatian: Zašto miješaš ovoliko smjese? - Why are you mixing this much batter?

Ovoliko means "this much" and is used when the quantity is physically close to the speaker. If the batter was over by the person you are talking to, you would use toliko ("that much"). If it was far away from both of you, you would use onoliko.

Why is the word for batter in the form smjese rather than the accusative smjesu?

Words expressing quantity, like ovoliko, require the noun that follows to be in the genitive case. This is often called the partitive genitive because it refers to a quantity or part of something. So, the base word smjesa becomes the genitive smjese.

The translation says "batter", but doesn't smjesa just mean "mixture"?

Yes, smjesa literally means "mixture" or "blend". In cooking contexts, it is the standard word used for a batter (like pancake batter) or a wet mixture of ingredients.

Why use the imperfective verb miješaš here?

Miješaš is imperfective, which fits perfectly because the person is currently in the middle of the action. You are asking "Why are you mixing this right now?". A perfective verb would focus on the completion of the action rather than the ongoing process.