Start by preparing your marinade in a large mixing bowl. Combine minced onion, garlic, ginger, soy sauce, rice vinegar, brown sugar, sesame oil, and black pepper. Stir until the sugar completely dissolves and taste for balance - you want sweet, salty, and tangy flavors.

Add the flanken-style short ribs to the marinade, ensuring all pieces are completely submerged. Cover with plastic wrap and refrigerate for at least 8 hours, preferably overnight. Flip the ribs halfway through marinating for even flavor distribution. Remove the marinated ribs from the refrigerator 30 minutes before cooking to bring them to room temperature. This ensures even cooking throughout the meat.

Heat a large skillet over medium-high heat and add vegetable oil. Working in small batches to avoid overcrowding, sear the ribs for 3-4 minutes per side until nicely browned. Transfer seared ribs to a lightly greased 9x13 baking pan.

Preheat your oven to 375°F while finishing the searing process. Pour a few tablespoons of the leftover marinade over the ribs in the baking pan, including any garlic and ginger pieces for extra flavor. Cover the pan tightly with foil and bake for 25-30 minutes until the meat is tender and cooked through. The internal temperature should reach 145°F for medium doneness.