That is caused by having too much petrol in the carborator when you start it up. Then as it runs for a second it burns off all the excess fuel and then the smoke should go away. There really isn't anything you can do about this because when you shut the lawn mower down the fuel will sit in the cylinders until the next time you start it up. You may need a tune up on your mower and if that doesn't work maybe you are putting to much oil in your gas mixture. I hope this helps.
If it's a mower that dosn't have oil and gas mixture, you may have a piston ring that is leaking a tiny bit of oil past it when you turn it off. I have a mower that is about 10 years old that does this. It's been doing it for about 5 years and as long as it dosn't get any worse, I'm not going to worry about it.
