Butterfly the chicken breast with a sharp knife.
Place each breast between two sheets of plastic and pound with a mallet to 1/4 inch thick.
Measure the length of each breast to 8 inches and the width to 5 inches.
Trim off any excess chicken and set aside.
Heat 1 tbsp of olive oil in a large skillet over med/hi.
Saute the onion until soft.
Add the garlic, spinach and 1 1/2 tsp of lemon juice to the pan and cook one minute more.
Place the mixture in a bowl and set aside to cool.
Add the 1/2 cup of wine to the pan and deglaze for a minute or so.
Pour the wine in a separate bowl to reserve for later.
Take the pan off the heat and set aside.
Place the chicken scraps that were trimmed off the breasts into a food processor along with the onion and spinach mixture.
Add the salt, pepper and thyme and pulse until the mixture is finely ground into a paste.
Spread a spoon full of the mixture at a time over each chicken breast until covered, leaving enough room around the edges to roll.
Roll each chicken breast into a log and secure with butcher's twine.
Cut the excess string off after the knots are tied.
Season the chicken well with salt and freshly ground pepper on the outside.
Place your pan back over med/high heat and add a tablespoon of olive oil.
Sear each chicken breast on all four sides, about 2 minutes per side.
Add the reserved wine and chicken stock to the pan.
Cover and allow the liquid to come to a simmer.
Turn the heat to med/low and cook for 15 to 18 minutes or until the internal temp of each chicken breast is 160 degrees F.
Remove the chicken and set on a platter to rest.
Cover with foil.
Turn the heat back up to med/hi and allow the liquid to come to a boil.
Add the Dijon mustard, 1 1/2 tsp lemon juice and butter.
Reduce the heat to med and simmer until the liquid is reduced by half.
Remove the twine from the chicken pieces and slice into medallions.
Serve with the reduced sauce poured over top.