Heat the oil in a Dutch oven or large soup pot over medium heat. Add the onion and cook for 4 to 5 minutes, stirring occasionally, until softened.
Stir in the garlic, ginger, turmeric, coriander, and optional red pepper flakes. Cook for 30 seconds, stirring constantly, until fragrant.
Add the lemongrass, coconut milk, vegetable broth, soy sauce, and brown sugar. Stir well and bring the liquid to a gentle simmer; do not let it boil vigorously.
Add the tofu and green beans. Simmer gently for 6 minutes.
Add the zucchini, cabbage, and corn. Continue simmering for 7 to 9 minutes, until the green beans are tender-crisp, the zucchini is tender, and the cabbage has softened.
Remove the pot from the heat. Stir in the lime juice, then taste the broth. Add a small extra splash of soy sauce if desired. Remove and discard the lemongrass before serving.
Ladle the stew into bowls, sprinkle with cilantro if using, and serve hot with cooked jasmine rice.