Squeeze sausage from casings into a medium sized mixing bowl.
Add the tomatoes, parmesan, garlic, egg, fennel, oregano, pepper, basil and salt to the sausage, and mix together.
With a large knife, cut the bottom 1/3 of the roast lengthwise without cutting through the opposite side. Continue to cut the remaining 2/3 of the roast in half lengthwise. The result should be one long, flattened rectangle of pork.
Spread the sausage mixture evenly across the pork, leaving a ½” border around the edge.
Press the crumbled feta cheese evenly into the sausage mixture.
Lay a single layer of spinach leaves over the entire surface of the sausage and cheese.
Gently roll up the roast in the same direction that it was cut to its original shape, making sure that it is not too tight. Tie the roast firmly with loops of the butcher’s cord every ½”-1” all the way across the roast. Finish tying with one more loop lengthwise to fully secure.
Season the outside of the roast liberally with salt and pepper.
Heat a large heavy bottomed pan over medium-high heat. Add the vegetable oil and sear the roast on all sides until completely browned.
Place the roast on a rack in a shallow baking pan, insert a meat thermometer, and bake until the internal temperature reaches 160⁰F, approximately 2 hours.
Remove from the oven and let it rest for 15 minutes before carving and serving.