回答如下:

Additional courses may be taken if desired.(if desired=if they are desired; they指代additional courses)

 如有需要可选修额外的课程。

Transfer this sauce to a separate saucepan, if desired. (if desired=if it is desired;  it指代前面分句整个内容)

如果愿意的话，可以把这些调味汁倒入一只单独的长柄锅里。

Purchasing additional named callers if desired.

买更多的名额。

Serve with Red Wine Sauce if desired.

如果喜欢可以搭配红酒酱食用。

Slice or cut kidney in pieces if desired.

如果需要将它切成片或条。

If desired, garnish with oregano; serve.

最后用牛至做装点，即可食用。

Serve on Arugula Salad if desired.

如果喜欢，可与芝麻菜沙拉一起食用。