Marinate the Beef: Mix the marinade ingredients in a bowl big enough to hold the beef. Add the beef and mix well. Use your hands to massage everything together. Set aside for 30 minutes, or longer if desired.
Prep the Ingredients: Prep the ingredients per the instructions above.
Make the Rice: Rinse the rice under cold water until water runs clear. Cook the rice according to the directions on the package. Once tender, season with salt and pepper to taste. Cover with a lid to keep warm until serving.
Make the Potatoes: In a bowl, mix soy sauce, mirin, garlic, sugar, and water until combined.
In a nonstick pan, heat the oil over medium-high heat. Sauté potatoes for 4–5 minutes until they begin to turn golden.
Add sauce and simmer until the potatoes are fork-tender but not mushy. If the sauce is too watery, turn the heat to high and let the sauce reduce until caramelized. Set potatoes aside to cool until serving.
Make the Beef: Heat a large skillet or nonstick pan over medium-high heat.
Remove the beef from the marinade, reserving the marinade.
Strain the marinade into a saucepan over medium heat and bring to a simmer. Slowly stir the dissolved cornstarch into the marinade a little at a time (you may not need it all) until the marinade thickens to a smooth consistency, then remove and set aside.
Add the oil to the heated skillet or pan and sauté the onions for 2–3 minutes.
Add the beef (in batches if needed) and pan-fry or stir-fry the meat until slightly brown on both sides. Avoid overcrowding the pan to prevent excess juices.
Pour the thickened marinade into the pan with the beef. Set aside until ready to serve.
Garnish: Garnish the potatoes with sesame oil and sesame seeds.
Serve: Serve the bulgogi with rice, kimchi, braised potatoes, and coconut water.