In a large skillet over medium-high heat, melt the butter and add the sausage. Cook until browned, about 5 minutes. Reduce the heat to medium and add the onion, red pepper, 1/2 teaspoon salt and 1/4 teaspoon pepper and cook, stirring occasionally, until vegetables are soft and lightly caramelized, about 8 minutes. Add the arugula and continue to cook until wilted, 2 to 3 minutes. Remove from the heat and transfer mixture to a colander set over a bowl and set aside to drain until cool enough to handle. When the mixture has cooled, transfer to a cutting board and roughly chop.

In a mixing bowl, combine the eggs, sour cream, bread crumbs, sausage mixture and parsley and mix well.

Using a paring knife and a knife steel, or other long, pointy object, work gently to make a hole through the middle of the tenderloin down its entire length, beginning at both ends by making cuts into the flesh using the knife, and then using the steel to further lengthen and widen the hole until it runs the length of the tenderloin and is wide enough for stuffing, about 1 1/2 to 2 inches wide. This will take some time and patience. You can also ask your butcher to help you with this if you are uncomfortable trying this at home.

Preheat the oven to 450 degrees F.

Using your fingers, carefully stuff as much of the stuffing as possible into the cavity, making sure to fill the entire cavity with stuffing. Any small amount of leftover stuffing may be placed in a baking dish and baked along with the fillet for serving on the side, if desired.

Using butcher's twine, tie the fillet every 3 inches with butchers twine to help the fillet hold its shape. Roast on a large baking sheet until browned on the outside, about 10 to 15 minutes. Reduce the heat to 350 degrees F and continue to cook 20 minutes longer for medium rare. Remove from the oven and set aside on a cooling rack to rest for 15 minutes before carving.

When ready to serve, remove the butcher's twine and discard. Cut the fillet into thick slices and serve immediately.