1. Grate potatoes and onion into a large bowl. Add some water just to cover the potatoes so that they don't brown while you are grating the rest. Let stand for 10 minutes to let the liquid from the potatoes and onion separate and rise to the top. Remove the liquid and then pat the potatoes and onions dry to remove any excess water. This helps them crisp up in the pan.
3. Heat the vegetable oil in a pan over medium-high heat. Oil in the pan should be about 1/4" deep. I like to test by putting a small pinch of flour in the pan when I think it is hot enough. If it sizzles the oil is ready to go.