If you're cooking for one person or two, it absolutely is lazy, unless the kitchen is a mess beforehand. But given that it's not, washing the knife and cutting board quickly right after you put the onions in the pan takes like 10 seconds, and it's time you would be standing there waiting anyway. You're not spending any extra time or doing almost any extra effort. And chopping itself will take you like a minute or two if you're going slow.