For that particular situation, I would cut the tile back to the exact size of a shallow Wiremold box:
Install the Wiremold box over the outlet box, then install the outlet and cover on the Wiremold box, and then caulk or grout the edges. (This would have been easier before the tile went down but can be done with some patience and maybe a Dremel tool to clean the corners up.)
The Wiremold backing plate should bolt right up to the electrical box you currently have installed. All you need to do is cut the tile back to a flush surface so it buts up against the surface mount box.