Prepare the Ingredients: Thinly slice the beef sirloin against the grain, mince the garlic, and slice the onion and red bell pepper.

Heat the Skillet: Add 2 tablespoons of vegetable oil to a large skillet over medium-high heat until shimmering.

Cook the Beef: Add the sliced beef in a single layer and cook for 3-4 minutes until browned, then transfer to a plate.

Sauté the Aromatics: In the same skillet, add the minced garlic and sliced onion, cooking for about 2 minutes until translucent.

Add the Bell Pepper: Toss in the sliced red bell pepper and cook for another 2-3 minutes until softened.

Make the Sauce: In a small bowl, mix 3 tablespoons of soy sauce, 1 tablespoon of brown sugar, 2 tablespoons of gochujang, 1 tablespoon of sesame oil, and 1 tablespoon of rice vinegar until smooth.

Combine the Sauce and Vegetables: Pour the sauce over the sautéed veggies in the skillet, stir well, and simmer for 1-2 minutes.

Return the Beef to the Skillet: Add the cooked beef back into the skillet and toss everything together for about 1 minute.

Incorporate the Noodles: Add cooked noodles to the skillet and toss gently to combine everything.

Heat Through and Serve: Allow to heat for another 2-3 minutes and serve garnished with sesame seeds and sliced green onions.