Preheat oven to 400 degrees F.
Quarter baby potatoes and cover with cold water. Bring to a boil then salt water and cook until tender, about 12 minutes.
While potatoes come up to a boil, place a nonstick baking sheet alongside a mixing bowl. Remove 12 pieces of bite sized mozzarella from the water and set on work surface. Remove sausages from casings, if bulk not available, and place in a bowl. Combine the meat into a single mound then score it into 4 sections. Each section will yield 3 large sausage meatballs. To form each ball, take a handful of meat and wrap it around a piece of mozzarella. Roll 12 balls. Wash up. Coat balls with a drizzle of olive oil. Bake 18 to 20 minutes until cooked through and evenly browned.
While the balls are cooking, trim ends of clean broccolini and scatter evenly over a baking sheet. Grate or chop 2 to 3 cloves of garlic and drizzle the broccolini and garlic with olive oil, then season with salt and pepper and roast 12 to 15 minutes to crisp the broccolini tops.
While the broccolini is roasting, add tomatoes to a small pot or skillet, mash with a potato masher and season with salt and pepper. Heat over medium heat then simmer to reduce about 8 to 10 minutes.
When the potatoes are tender, drain and return to the hot pot to dry them out a little. Mash the potatoes with 1/2 cup stock or milk, 1/2 to 2/3 cup grated Romano cheese, a few generous handfuls, lots of black pepper and a little salt.
Turn the heat off the tomatoes and stir in 1/2 cup prepared pesto.
Serve three cheese-stuffed sausage balls with tomato-basil sauce on top or underneath, cacio e pepe smashed potatoes and broccolini alongside.