let ... sit together
rhubarb（ダイオウ）を用いた特製ジャムを紹介するMainichiにあった記事から英語表現を拾います。
sitはto be in a particular position or condition（LDOCE）とあるように「ある状態のままにしておく」という意味があり，後続のfor a whileと合わせて料理の「寝かせる」にあたります。togetherは「混ぜ合わせた状態」を表し，以上からletting the ingredients sit together for a whileは「混ぜた状態でしばらく寝かす」と解釈できます。（Koyamamoto）
Farmer's unique rhubarb jam packed with flavor
With spring vegetables coming into season, the Mainichi Shimbun introduces one farmer's method to enjoy rhubarb and other vegetables in a new way: making them into jams.
... The basic recipe is to simply cut rhubarb stems into 2-centimeter pieces, mix them with coarsely-grained sugar and boil them in a pot. It's recommended to use about half the weight of the rhubarb stem for the volume of sugar. After applying a little lemon and letting the ingredients sit together for a while, stir it on high heat for around 15 to 20 minutes until a greenish jam appears.
http://mainichi.jp/english/english/features/news/20140506p2a00m0na009000c.html
