Begin by thinly slicing 1 pound of beef sirloin against the grain for tenderness. Mince 3 cloves of fresh garlic and slice 1 medium onion and 1 red bell pepper into bite-sized pieces. Meanwhile, cook your choice of noodles according to package instructions, usually around 6-8 minutes, until al dente.

In a large skillet, heat 2 tablespoons of vegetable oil over medium-high heat. Once the oil shimmers, add the sliced beef and cook for about 3-5 minutes, stirring occasionally, until browned and cooked through. The beef should reach an internal temperature of 145°F.

Using the same skillet, add the minced garlic and sliced onion, stirring frequently for about 2 minutes until fragrant and the onion is translucent.

Next, add the sliced red bell pepper to the skillet and stir-fry for 2-3 minutes until it softens slightly.

In a bowl, combine 1/4 cup of soy sauce, 2 tablespoons of brown sugar, 2 tablespoons of gochujang, 1 tablespoon of sesame oil, and 1 tablespoon of rice vinegar. Whisk these ingredients together until well blended.

Pour the sauce mixture into the skillet with the sautéed vegetables, stirring to coat everything evenly for about 1-2 minutes.

Return the cooked beef to the skillet, tossing it to coat it with the sauce and vegetables. Then, add the drained cooked noodles, mixing everything thoroughly for 2-3 minutes.

Transfer your Spicy Korean Gochujang Beef Noodles to serving plates, garnishing with toasted sesame seeds and sliced green onions for a fresh touch.