Heat a large Dutch oven or wide shallow pot on medium heat for 2 minutes. Add the sesame oil and onion and stir until softened, 3–4 minutes.
Add the gochujang, white parts of the spring onion and the garlic and stir for 1 minute.
Add the mushrooms, kimchi (along with any brine) and 1 teaspoon salt. Cook, stirring occasionally, until the mushrooms are softened, about 4–5 minutes.
Add the vegetable stock and the tofu and gently combine, being careful not to break the tofu into small pieces — ideally, the tofu should be in large chunks. Cover and cook until the tofu is warm and flavors have melded, 15–20 minutes. When ready, turn off the heat, add the green parts of the spring onion.
Taste and if needed, season with salt. Serve with rice.
