Place a rimmed baking sheet on the middle oven rack and preheat to 450˚ F. Toss the brussels sprouts and potatoes with 1 1/2 tablespoons olive oil and 2 smashed garlic cloves in a large bowl; season with salt and pepper. Spread out on the hot baking sheet and roast until tender and browned, 25 to 30 minutes.
Meanwhile, season the pork all over with salt and pepper, then rub with the rosemary and grated garlic. Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the pork and cook, turning, until browned all over, about 6 minutes. Transfer the pork to a small baking sheet (reserve the skillet) and roast in the oven until cooked through, 10 to 12 minutes. Remove to a cutting board. Let rest 5 minutes, then slice against the grain.
Meanwhile, add the remaining 1/2 tablespoon olive oil to the reserved skillet; heat over medium-high heat. Add the red onions and a big pinch of salt and cook, stirring occasionally, until softened and lightly browned, about 6 minutes. Reduce the heat to medium and add the balsamic vinegar, 1/2 cup water and the sugar. Simmer, stirring occasionally, until the sauce thickens, 8 to 10 minutes; season with salt and pepper.
Divide the pork, onions and roasted vegetables among plates. Sprinkle with the parsley.