すみません、豚バラ肉をもっと薄くスライスしてもらえますか？ — Excuse me, could you please slice the pork belly thinner? (Supermarket · Butcher, Formal — to staff, JLPT N3)

Gesture & etiquette

Point to the meat. Use fingers to indicate desired thickness if words fail (though 'usuku' is clear).

Be specific about the cut and thickness. Be prepared for them to say no if they don't have the equipment or time. In some supermarkets, this service might not be available.