Heat 1 tbsp neutral oil in a small heavy pot over medium heat and cook the sliced pork until the edges brown, about 3 minutes.
Turn the heat to low, add the gochugaru and minced garlic, and stir constantly for 30 to 45 seconds until the oil turns deep red and smells toasty. Do not let it darken past that or it will taste burnt.
Add the onion and chopped kimchi and cook 2 minutes, until the onion softens.
Pour in 500 ml / 2 cups stock, raise the heat, and bring to a boil. Simmer 5 minutes.
Open the tube of sundubu and squeeze it into the pot in large pieces. Do not stir hard — nudge the curds apart with a spoon so they stay in soft clouds.
Simmer 4 to 5 minutes, until the tofu is heated through and the broth is bubbling at the edges.
Season with 1 tbsp soy sauce and 1 tsp fish sauce. Taste and adjust with salt.
Crack the egg into the center, kill the heat, and let it sit 60 seconds so the white just sets. Scatter scallions and finish with 1 tsp toasted sesame oil. Serve straight from the pot with hot rice.