re: Oops

splint has the 2 best answers and just looking at that one picture I'm not too sure which way will satisfy.  
However, my first instinct would be to put a 3/4" straight router bit in the router table and "take off" as much as needed to get to 'good wood'  and allow another walnut border to be glued in place.  I see you already have a walnut trim, you can accessorize with a second walnut strip and call it design.  I would make it bigger in height and width than needed and either rip off the excess on the TS or router table then finish sand to meet the host wood.
Probably doesn't make too much sense, but think about it a while, it will then.

btw - some of my neatest designs come from exactly what you have