Preheat oven to 400 degrees F. Grease a 9x13 baking dish.
Cut pork tenderloin lengthwise nearly all the way through, so that you can open it like a book, leaving a “hinge” on the end. Brush flat surface with 5 tsp olive oil. Sprinkle with parmesan, 2 ½ tsp garlic, the onion powder, italian seasoning and a pinch of salt. Add spinach and shredded mozzarella in an even layer. Close like a book. Tie shut with kitchen twine if available. Spread mustard over the top.
Place pork tenderloin in the prepared baking dish and bake 45 minutes, or until a meat thermometer reaches 145 degrees in the center.
Meanwhile, with a vegetable peeler, spiralizer or sharp knife, shave zucchini lengthwise into thin ribbons.
When pork is nearly ready, heat remaining 10 tsp olive oil in a skillet over medium heat. Add remaining 5 tsp garlic and the red pepper flakes and cook 1 minute. Add zucchini and stir to coat. Sprinkle with additional parmesan to taste.