Prepare the onion rings by peeling a large onion and slicing the ends off. Cut the onion through the middle and separate the largest rings from each half, with each ring being about 2 layers thick. Each serving will need 1 double-layered ring about 1 -¼ oz in weight. Rub ¼ teaspoon of olive oil all over each onion ring and season the rings with salt and pepper. Arrange the rings on a well-seasoned sheet tray with the wider parts of the rings facing up.
In a mixing bowl, combine ground Italian sausage with mayonnaise and garlic powder. You can use either ground Italian sausage that is packaged or you can remove the casing from links of sausage. Simply use a clean hand to mix the meat, mayonnaise, and seasoning until you have a consistent mixture. Gently press the Italian sausage mixture into the prepared onion rings until each serving is filled to the top of the rings. Lay a slice of cheese over every portion, and set this tray aside in your refrigerator until later.
Fill a pot with water and season the water with at least 1 teaspoon of salt. Bring the water to a boil on the stove, and add whole spinach leaves to the water. Boil the spinach for just 1-2 minutes until the leaves turn bright green and soften slightly. Then, drain all the water from the spinach. At this point, turn on the oven to preheat to 400 degrees.
Use a pair of tongs to arrange the spinach into 1 portion per serving on a second seasoned sheet tray. To build a portion, pick up the spinach with the tongs, let the excess water drip off, then use the tongs to spread the spinach into a flat circle. Top each portion of spinach with about ½ tablespoon of ground pork rinds, ½ teaspoon of parmesan cheese, and ¼ ounce of diced pancetta.
Place both the tray of spinach and tray of stuffed onions in the oven. Bake the spinach tray for 10-15 minutes and the stuffed onion tray for 25-30 minutes. For the spinach, look for golden and crispy prosciutto, darkened pork rinds, and slight browning on the spinach. For the stuffed onions, the cheese will be bubbly and browned and a meat thermometer should read at least 160 degrees. Serve a stuffed onion with a portion of spinach using a spatula.