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

Pronunciation: Zhuh voo-dreh kom-ahn-day uhn plah doo zhoor ah-vek oon sohs ah par.

Meaning: I would like to order the daily special with the sauce on the side.

This phrase demonstrates both politeness with the conditional "je voudrais" and practical dining knowledge—asking for sauce separately (à part) is a common French dining practice for those with dietary preferences.