Meatball Sliders

* 1 1/2 lbs. ground beef, pork, veal - mixed or all one type
* 1/2 c. panko (Japanese breadcrumbs)
* 1/2 c. water
* 8 tbl. freshly grated Pecorino Romano cheese, divided
* 1 lg. egg
* 1 lg. egg yolk
* 1/4 c. plus 2 tbl. chopped fresh parsley
* 1 tsp salt
* 1/2 tsp. ground black pepper
* 1/4 c. vegetable oil
* 2 Tbl. olive oil
* 1 c. chopped onion
* 6 garlic cloves, chopped
* 1/4 c. (packed) fresh basil leaves
* 1 1/2 tsp. fennel seeds
* 1- 28 oz. can whole peeled tomatoes
* 1- 14.5 oz. can whole peeled tomatoes
* Arugula leaves (optional)
* 18 sm. soft rolls, split

Mix all meats, panko, 1/2 c. water, 6 Tbl. cheese, egg, yolk, 1/4 c. parsley, 1 tsp. salt & 1/2 tsp. pepper in lg. bowl. Form into 18 2-in. meatballs. Heat oil in large skillet over med-high heat. Working in batches, fry meatballs until brown all over. Transfer to plate. Pour off drippings from skillet. Add onion, garlic, basil & fennel seeds. Saute until onion begins to brown, about 5 minutes. Add all tomatoes with juices. Bring to boil, scraping up browned bits. Reduce heat to low, cover with lid slightly ajar, & simmer stirring occasionally, about 30 minutes. Puree sauce in processor until almost smooth. Return to same skillet. Add meatballs. Cover with lid slightly ajar & simmer until meatballs are cooked through, stirring occasionally, about 30 minutes longer. (These can be made a day ahead!) Place arugula leaves on bottom of each roll, if desired. Top each with 1 meatball. Drizzle meatballs with some of sauce & sprinkle with remaining 2 Tbl. parsley & 2 Tbl. cheese. Cover with tops of rolls.