If using pork, brown it in a dry pot over medium heat for 2 minutes before adding anything else. If using clams or going meatless, skip straight to the next step.
Add the stock to the pot and bring to a simmer.
Scoop the doenjang into a small ladle or fine sieve, lower it into the simmering stock, and stir with a spoon to dissolve it against the mesh. This keeps lumps of undissolved paste out of the final stew.
Add the potato first and simmer 4 minutes, since it takes the longest to cook through.
Add the onion and zucchini, and the clams here if using. Simmer another 4 minutes.
Add the tofu, garlic, gochugaru if using, and sliced chili. Simmer gently 3 to 4 minutes — a hard rolling boil at this stage makes the broth taste flatter and slightly bitter, so keep it at a low, steady bubble.
Taste and adjust. Doenjang brands vary widely in saltiness, so add more paste or a splash of stock rather than following the measurement blindly.
Scatter the scallion over the top and serve immediately, straight from the pot, with a bowl of rice on the side.