Melt the butter in a large skillet over medium heat. Add the scallion whites and cook until wilted, 1 to 2 minutes. Whisk in the wasabi paste, then add the peas and vegetable stock. Bring to a simmer, then cover and cook until the peas are heated through and bright green but still saucy, about 4 minutes. Uncover, then season with 1/2 teaspoon salt and stir in the lime zest, cilantro and scallion greens. Spoon into a serving bowl and sprinkle with the wasabi peas.