Melt the margarine in a large skillet over high heat. Add half each of the onions, celery, and bell pepper, and saute until lightly browned.
Add the remaining onions, celery, and bell pepper, along with the andouille, butter, and Tabasco. Saute about three minutes.
Add the chicken stock and bring to a boil. Reduce to a simmer and cook until most of the fat rises to the surface; skim this off, and cook until all the liquid has been absorbed, but the mixture is still moist.
Stir in the bread crumbs to combine completely. Turn the mixture out onto a sheet pan to cool.
With a knife-sharpening steel or the handle of a wooden spoon, push a hole into the large end of the pork tenderloin, going in as deep as you can. Pack this hole with the cooled stuffing. (You will be surprised how much the meat will stretch, and how much stuffing will fit, but it does take a little patience.)
Heat up a grill or a large black iron skillet, and sear the outside of the stuffed tenderloins. Do not cook through yet. You can prepare the dish up to this point and hold it in the refrigerator for later use the same day. (In fact, this helps the stuffing to set.)
To finish the stuffed pork, slice the tenderloin into discs about three-fourths inch thick. Coat lightly with flour, salt, and pepper.
Heat the olive oil in a skillet on top of the stove and put the pork discs in. Put the skillet into a preheated 400-degree oven, and cook until medium--about six minutes--turning once.
Make the sauce in the skillet in which the pork was roasted. Melt 1 Tbs. of the butter, and saute the garlic and rosemary for about two minutes. Add the veal stock or demi glace. Bring to a boil, then remove from the heat. Whisk in the remainder of the butter to create an emulsified sauce.