Hearty cabbage combined with the lightness of lemongrass yields a sunny, refreshing soup - and you can eat a lot of it without feeling stuffed. The potatoes add a little body. If you prefer sweet potatoes, use them instead of Yukon Golds. The soup freezes well.
Heat a stovetop pressure cooker over medium heat or set an electric cooker to saute; add the oil, if using. Add the onion and cook for 1 to 2 minutes, until no longer raw. Add the chile, if using, the lemongrass, garlic, and ginger, and cook another minute or two until the onion is just beginning to soften.
Add the sliced cabbage and potato, along with the stock and coconut milk. Lock on the lid. Bring to high pressure; cook for 4 minutes. Let the pressure come down naturally. After 10 minutes, quick release any remaining pressure. Carefully remove the lid, tilting it away from you.
Using tongs, remove and discard the dried chile, if using, along with the ginger and lemongrass pieces.
Add lime zest and juice to taste and season with salt and pepper and grated ginger, if you like. Garnish with cilantro and serve.