To make the meatballs, in a large sauté pan, warm just enough olive oil to coat the bottom of the pan. Add the onion and cook, stirring occasionally, until translucent, about 3 minutes. Add the garlic and cook until fragrant, about 30 seconds more. Transfer to a small bowl and let cool.

In a large bowl, combine the ground beef and pork. Add the panko, cheese, egg, parsley, onion mixture, salt and pepper and mix thoroughly. Fry a little of the meat mixture and taste, adding more salt, if needed.

Using a 1/4-cup measuring cup, scoop out the meat mixture and roll into meatballs. You should have about 18 meatballs.

In the same pan used to cook the onions, warm more olive oil over medium-high heat. Add the meatballs and cook until browned, 8 to 10 minutes. Transfer to a paper towel-lined plate.

To make the sauce, in the same pan used to brown the meatballs, cook the onion over medium-high heat until translucent. Add the mushrooms and cook for 3 to 5 minutes, adding more olive oil if needed. Add the garlic and cook until fragrant, about 30 seconds more.

Add the wine, stock, tomato paste and tomatoes with juices and stir to combine. Bring the mixture to a boil, then reduce to a simmer. Add the meatballs, cover and cook until they are cooked through, about 25 minutes. Uncover and add the thyme, rosemary, oregano and basil. Increase the heat to high and cook until the liquid is reduced by half.

Remove the pan from the heat and stir in the mustard. Add the pepper and adjust the seasoning with salt.

Split the rolls in half. For each slider, place 1 meatball with some sauce on the bottom half. To gild the lily, grate more Parmesan over the sauce or add a slice of fresh mozzarella. Cover with the top half of the roll and serve immediately.