In a large straight-sided pan, heat the olive oil with the sausage over medium heat. Brown the sausage, breaking it up into smaller pieces as it cooks, until no longer pink, 8 to 10 minutes. Add the onions and turn the heat down to medium-low. Cook until the onions are fully softened, about 8 minutes. Season with salt and pepper.
Bring the chicken stock to a simmer in a saucepan and keep warm on low heat. Add the rice to the onions and increase the heat to medium. Toast the rice, stirring often, until fragrant, 1 to 2 minutes. Add the white wine and reduce until almost dry. Reduce the heat to medium, then add 6 ounces of hot stock at a time, stirring the risotto throughout. When the liquid evaporates, add more stock. Continue in this manner until the rice is al dente, then cook out most of the liquid until thick. Turn off the heat and add the Parmesan, chives and butter. Spread the mixture on a sheet tray lined with parchment to cool completely in the refrigerator, 3 to 4 hours, or up to overnight.
Roll the risotto into 1-ounce balls, to make about 35 in total.
Preheat several inches vegetable oil in a pot or fryer to 350 degrees F.
Roll the balls in the flour, followed by the egg, then the panko. Fry the arancini in batches until golden brown, 3 to 4 minutes. Transfer the arancini to a paper towel-lined plate to drain any excess oil. Place the arancini on a platter and garnish with Pecorino-Romano and chives and serve with the spicy marinara and/or the Chimichurri, if desired.
Preheat a grill to medium-high heat. Lightly dress the scallions with 1 tablespoon of the olive oil and sprinkle with salt and pepper. Grill, flipping as needed, until charred and wilted, about 8 minutes. Chop the scallions really well and add to a bowl with the remaining olive oil. Fold in the sherry vinegar, chives, parsley, shallots and garlic.