Je voudrais commander un plat du jour avec une sauce à part.

Pronunciation: zhuh voo-dreh kom-ahn-day uh plah doo zhoor ah-vek oon sohss ah par

Meaning: I would like to order the dish of the day with the sauce on the side.

This phrase is practical for restaurants and shows politeness while expressing a common dietary preference—asking for sauce separately is very normal in French dining culture.