Bring the meat up to room temperature for 45 minutes. Pat dry with a paper towel to absorb excess moisture and encourage browning. Heat 2tsp of oil over a medium-high heat in a heavy-based cast iron pot. Brown the beef for 2-3 minutes each side. Set aside on a plate to cool. When cool enough to handle, dice into cubes roughly 2-3cm in size.
Heat the remaining oil in the pot and fry the onion until translucent (4-5 mins). Stir in the Gochujang paste, garlic, Korean Gochugaru Blend, soy sauce and sugar and cook until fragrant (2-3 minutes). Deglaze the pot with a splash of water, scraping the bottom as you go.
Return the meat (and any juices) to the pot and pour in the salt water (or broth). Slowly bring to a gentle boil over a medium-high heat. Once boiling, cover (with the lid slightly ajar), then reduce to low and simmer until the meat is tender (around 2.5 hours).
Soak the mixed forest mushrooms in hot water until plump (30 minutes). Drain and set aside the liquid for later use.
After 2.5 hours, temporarily increase the heat and add the potatoes and mushrooms. Once bubbling gently, simmer partially covered until the potatoes are cooked through (around 30 minutes). If the stew is too thin for your liking, mash some of the potatoes to thicken it. Alternatively, if it is too thick, adjust the consistency with the liquid from the mushroom.
Serve with steamed rice and freshly cooked broccoli or snow peas.
If you are using tofu instead of beef, alter the recipe as follows: soak the mushrooms first (instead of at step 4), then start at step 2.
At step 3, add the tofu, potato and mushrooms at the same time and cover with 500mL of water or vegetable broth instead of 750ml. Simmer with the lid off for at least 30 minutes or until the potatoes are cooked through.