Mise en place: Prep all ingredients and have them ready (see notes for toasting bread cubes if making your own)
Heat 1 Tbsp of butter in a skillet until melted and sizzling. Add sliced mushrooms and shallots. Cook and stir for several minutes, until shallot is softened and mushrooms have released most of their liquid.
Add minced garlic to your pan and stir just until garlic is fragrant (about 30 seconds).
Add spinach to pan and, using tongs, toss/turn spinach with mushroom mixture until spinach is all wilted and most of it's liquid is evaporated.
Add toasted bread cubes, stir, and season to taste with salt, pepper, thyme and red pepper flakes.
Once everything is coated and seasoned add lemon juice and enough broth to just to moisten. Continue cooking and stirring 1 minute. You may need to add another Tbsp of broth if mixture gets dry.
Remove filling to a bowl and set aside to cool.
To Assemble
Lay chicken out flat, skin side down, on work surface and lay sliced prosciutto evenly over entire surface to cover, overlapping slices slightly
Spread spinach/mushroom filling over prosciutto leaving 1/2-1" border all around
Top spinach mixture with grated gruyere and dot with butter (use your fingers to pull butter apart into little pieces)
Gently roll chicken up into a cylinder from the side (keeping breast meat at one end and thigh meat at the other. As you roll push/tuck prosciutto around filling on the open ends to keep everything inside.
One your chicken is rolled, use butcher twine to tie roast to keep it's shape (see notes for link to video on tying a roast). Season all over with kosher salt and cracked pepper
Cooking
Melt 2 Tbsp of fat in a heavy, ovenproof skillet (I use cast iron) and sear meat on all sides, turning as each side gets golden.
Once meat is completely seared, add smashed garlic cloves, lemon slices, fresh thyme sprigs (if using) and 1/4 cup chicken stock or white wine to juices in skillet.
Place in a preheated 400 degree f oven and roast for approximately 45 minutes, or until internal temperature reads 160 f. (ovens vary, as do the size and starting temperature of your chicken, so go by internal temperature rather than strictly by time)
Remove roast from pan and place on cutting board, covered with foil, to rest 10-15 minutes before slicing
Smash garlic and lemon into pan juices, strain and spoon over sliced meat before serving, if desired
