Cortar/rebanar algo a lo largo.

To cut/slice something lengthways.
To cut/clice something lengthwise.

Cut the avocado lengthways.


On Websites:

"When a recipe calls for sliced onions, you have a choice of how to slice them—lengthwise"
Simply Recipes.

In Dictionaries:

"Cut the pickles lengthwise."
Cambridge Dictionary Online.