In a large skillet over medium-high heat, add 2 tablespoons of oil and the onions. Saute until the onions are translucent. In the same skillet, add the sausage meat and cook until all the sausage has changed color and is cooked, about 5 minutes. Add the cornbread and fresh herbs and continue to cook for another 3 to 4 minutes. Remove from heat and allow to rest until cool.
When cool add the whipped egg whites, heavy cream and melted butter and stir to combine. Adjust the seasoning with salt and pepper, to taste.
Make a cut 1/4-inch from the top of the loin down the full length and roll the pork cutting at the same time, this will give you a flat piece of meat ( if you were to roll it back up it would look like it did before you cut it). Evenly spread the stuffing over the flattened pork loin and then re-roll it back to the original form.
Tie together with butcher's twine, so the loin is tied every 1 to 2 inches. Refrigerate for about 1 hour to rest and firm up a bit.
Preheat the oven to 400 degrees F. Using a large skillet over high heat, add the remaining 2 tablespoons of grapeseed oil and sear the whole pork loin until golden brown all over, then transfer the pan to the oven and bake for 15 to 17 minutes. Reduce the heat to 350 degrees F.
Check the pork temperature after 10 minutes, as some ovens cook faster than others, and we don't want the meat to overcook. The loin should be removed from the oven when it reaches an internal temperature of 135 degrees F on an instant-read thermometer. Transfer the loin to a cutting board and cover with aluminum foil, to rest for 5 minutes. It will continue to cook and reach an internal temperature of 145 degrees F. Slice the pork and arrange on a serving platter to serve.