This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. The reading for 素 is the そ reading, so be careful. You can remember by thinking how you have to sew the building together using these raw materials.
Aya was right in her decision to go with that raw material.
In this restaurant, seasoning is kept to a minimum for you to enjoy the natural taste of the ingredients.