Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Halve the Brussels sprouts and toss them in a bowl with 1 tsp of olive oil, smoked paprika, garlic powder, sea salt, and black pepper.
Spread the seasoned sprouts on the baking sheet and roast for 20 minutes until the outer leaves are crispy and charred.
While the sprouts roast, thinly slice the yellow onion into half-moons.
Heat a heavy skillet over medium-low heat with the remaining 1 tsp of olive oil and add the onions.
Sauté the onions for 15 minutes, stirring frequently, until they become soft and reach a deep golden brown color.
Stir the apple cider vinegar into the onions to deglaze the pan and scrape up any flavorful browned bits.
Slice the lean, pre-cooked brisket against the grain and add it to the skillet with the onions for 3 minutes to heat through.
Serve the smoky brisket slices topped with the jammy onions alongside the roasted Brussels sprouts.