Fill a medium pot 3/4 of the way up with salted water; cover and heat to boiling on high. Wash and dry the fresh produce. Cut off and discard any stems from the fennel, then halve, core, and medium dice the bulb. Thinly slice the scallions, separating the white bottoms and hollow green tops.

In a bowl, combine the sausage and breadcrumbs; gently mix to combine. Shape the mixture into 10 tightly packed meatballs. In a large pan (nonstick, if you have one), heat a drizzle of olive oil on medium-high until hot. Add the meatballs. Cook, turning occasionally and shaking the pan (carefully, as the oil may splatter), 5 to 7 minutes, or until lightly browned on all sides.  

Meanwhile, add the pasta to the pot of boiling water. Cook, stirring occasionally, 8 to 10 minutes, or until tender. Turn off the heat. Drain thoroughly and return to the pot. Add the butter and a drizzle of olive oil; stir until combined and the butter is melted.

Meanwhile, add the diced fennel to the pan of browned meatballs; season with salt and pepper. Cook, stirring occasionally, 2 to 3 minutes, or until slightly softened. Add the sliced white bottoms of the scallions. Cook, stirring frequently, 1 to 2 minutes, or until softened. Add the vinegar (carefully, as the liquid may splatter). Cook, stirring frequently and scraping up any browned bits (or fond), 30 seconds to 1 minute, or until the liquid has cooked off. Add the tomato sauce (carefully, as the liquid may splatter). Cook, stirring occasionally, 3 to 4 minutes, or until the sauce is thickened and the meatballs are cooked through.* Turn off the heat. Taste, then season with salt and pepper if desired. Serve the finished pasta topped with the finished meatballs and sauce. Garnish with the parmesan and sliced green tops of the scallions. Enjoy! 

*The USDA recommends a minimum safe cooking temperature of 160°F for pork.