The recipe your making lists "onion" in the ingredients, but which type do you use?
Unless the recipe lists a specific type by color or name, opt for a standard yellow onion. Yellow onions are tasty, cheap, and versatile; and tend to be neither too sharp nor too sweet.
Notes on a few other varieties:
Spanish: Spanish onions tend to be very large, thought their flavor is similar to yellow onions. Because this variety is so big, it isn't usually a practical choice for home cooking. Though if you are hosting a cookout or making an onion blossom, these could be a good choice.
Red: Red onions tend to be sweeter than yellow onions, and they add a nice pop of color to dishes you use them in, especially salads.
White: White onions are more pungent (i.e. likely to make you cry as you are cutting them) and have a fuller flavor than yellow onions. Depending on your taste preference, you can often interchange white and yellow onions without any problem.