Most people I have ever seen make a ganache avoid the problem by not starting out with melted tempered chocolate.There's no technical requirement to use melted tempered chocolate to make a ganache.
Of course, Cathy's answer is a very good one if, for some reason, you feel it's important (or easier) to use melted tempered chocolate. A cold bowl can precipitate crystal formation on the surface of the bowl before the bowl warms up. Slightly warming the surface of the bowl (with a hairdryer, as Cathy says - there are other ways) will keep that from happening.
Technically, there's also no requirement to even use melted chocolate. The heat from the cream or whatever liquid you're using to make the ganache should be enough to melt the (solid tempered) chocolate and cool the liquid down at the same time. If you do want to use melted chocolate then you can use melted chocolate, not tempered melted chocolate.
Another way: pour the warm liquid into the bowl and allow it a few seconds to warm the bowl. Then pour the melted tempered chocolate into the liquid. No separate warming step required.
Another thing you might want to consider is the fact that the heat of the mass of the warm chocolate with the warm liquid should be enough to remelt any chocolate crystals that have formed on the cool bowl. Thoroughly scraping the bowl with a spatula to dislodge any crystals and work them into the warm mass should melt them. Just let the mass sit for a minute before starting to mix, starting in straightaway with a spoon or balloon whisk won't do it.
You might also consider using an immersion blender. I use one for all my ganaches as it significantly reduces the risk of the ganache breaking. The blender does a much better job of creating a stable emulsion than mixing by hand in my experience. The immersion blender should be able to incorporate any small seeds evenly in the ganache.
clay - http://www.thechocolatelife.com/clay/