Preheat oven to 350 degrees.
In a large skillet, saute onions in olive oil until caramelized. Then add next eight ingredients (through red pepper flakes) and saute until liquid has evaporated. Add cheese, nutmeg, and salt & pepper and stir to combine.
Cool stuffing before filling turkey tenderloins.
Place turkey tenderloins, one at a time, between two sheets of plastic wrap. Pound gently to an even thickness of about a 1/2 inch. Place spoonful of stuffing at one end and roll tenderloins. Tie with butcher's twine in 3-4 places. Lightly brush tenderloins with egg wash or olive oil, then roll in panko breadcrumbs.
In a large oven-proof skillet, brown tenderloins in olive oil, then transfer to oven. Bake for 30 - 35 minutes, until internal temp. reaches 165 degrees. Remove and allow to rest for 10 minutes before slicing. Don't forget to gently remove the butcher's twine before serving.