Whisk the gochujang, gochugaru and doenjang, if using, with a splash of the stock until smooth — this keeps the paste from clumping once it hits the pot.
Cook the pork in a small pot over medium heat 2 to 3 minutes, until the edges brown and some fat renders out.
Add the garlic and onion and cook 1 minute, until fragrant.
Add the potato and the stock, bring to a boil, and simmer 8 minutes — the potato needs the longest cooking time of anything in the pot, so it goes in first.
Stir in the gochujang mixture, then add the zucchini and simmer 4 minutes.
Add the tofu and chili pepper and simmer 3 more minutes without stirring hard, so the tofu stays in cubes instead of breaking apart.
Taste and season with salt a pinch at a time — the gochujang already carries most of the salt the stew needs.
Scatter with scallion and serve bubbling, straight from the pot, with rice.
