Make the sliders: Mix the beef, sausage, bread crumbs, onion, parsley, garlic, egg, pepper, and salt together in a large bowl. Make a meatball with about 2 ½ ounces of meat. Place a half mozzarella ball in the middle and close the meat around it. Flatten slightly with your hand to resemble more of a fat hamburger than a round meatball.
Broil until cooked, turning once. About 3 minutes per side.
Place the meatballs into the marinara sauce that has been heated in a large skillet. Turn to coat. Allow to simmer in the sauce for a few minutes.
Lightly coat the cut side of the bun with olive oil. Heat a cookie sheet in a 375º F oven, place cut side down on the baking sheet and bake until the cut side is just beginning to brown, but the bun is still soft; about 5 minutes.
Place the meatball onto the bun, add a little marinara over the top. Spread the top of the bun with some of the pesto, top the meatball with a spinach leaf, some arugula or a basil leaf, and top with the bun. Serve immediately.