You don't actually burn weeds off, you're just wasting gas. What you do is briefly aim the flame at the base of the weeds/grass. Just till you see the green turning a few shades darker. By doing that you disrupt the cells that are needed for the weed to survive and it will shrivel up over the next week or so. Once died back, a second go with a flame gun , after a few dry days, will burn the remains off. If you're going anywhere near your onions with heat you'd be well advised to use a guard to protect them.
