Slice the pork tenderloins lengthwise, unrolling each one like a roll of wrapping paper. Wrap loosely in plastic wrap and gently pound to an even thickness. Lightly season the cut side with desired amount of salt and pepper and set aside.
Melt six tablespoons of butter in a large skillet over medium heat. Add the onions and cook until softened, about four minutes. Add the apples and cook until they just start to soften, about two minutes. Stir in the minced sage, cinnamon, and cloves. Remove from heat, and let cool slightly. Then stir in the Dijon and lemon juice. Season lightly with salt and pepper to taste.
Divide the stuffing evenly among the four tenderloins and spread a thin layer over the surface of the cut sides. Roll up gently and tie with butcher’s twine every one to two inches, being sure to tie each end first, so as not to force the stuffing out of the end of the roll. Brush the outsides with olive oil and season with salt and pepper.
Preheat the oven to 375°F. While the oven is preheating, heat the remaining olive oil in a large stainless steel skillet (or two if needed) over medium-high heat. Sear each stuffed tenderloin on all sides until golden brown.
Transfer the skillet to the oven and roast until each tenderloin reaches desired internal temperature, about 15 minutes depending on size. Remove and let rest for 5 to 10 minutes before slicing.
While pork is resting, melt six tablespoons of butter in a pan over medium heat. When butter starts to foam, and you see brown bits forming in the bottom of the pan, add the whole sage leaves and let them sizzle until crisp, about 30 seconds. Turn off the heat and season with salt and pepper.
Slice the tenderloins and spoon the brown butter over the top, letting the crisp sage leaves rest on each serving.