The use of verb stem in the sentence

Question

Why in this sentence the stem of the verb is used instead of the verb 炒める itself?

まず、玉葱を炒め、次に肉を入れて下さい。

Answer 1

Most often in literary or written formats, you will see the 'verb stem' used in this way as a less 'colloquial' alternative to the て-form. So your example can also be rendered as follows:

まず、玉葱を炒めて、次に肉を入れて下さい。

See also this thesis entitled "On Japanese Coordinate Structures: An Investigation of Structural Differences Between the -te and the i-form", if you are interested to read a rather more detailed treatment on the subject!

Hope that helps!