There are two possible causes of the smell of sewer gas: 1) The waste plumbing is not properly trapped and vented. The trap below the sink prevents sewer gases from entering, because it is full of water. The waste plumbing is also vented to allow water to flow freely and to prevent the water from being siphoned from the trap.
If your bathroom was professionally remodelled, there shouldn't be a problem with the waste plumbing configuration. 2) The more likely scenario is as your plumber suggests. For some reason, a more than average amount of foreign material is getting into the overflow tube that runs from the overflow hole in your sink, to the drain at the base of your sink. Water flowing down the drain forces air back up though the overflow. Decaying material in the overflow pipe then gives off a smell. (This is why kitchen sinks don't have an overflow. Food particles caught in the overflow would smell.) This is why the bleach down the overflow worked. You can verify this by temporarily taping over the overflow and running water. The solution is to use bleach regularly or prevent foreign material from entering the overflow.