In a large saucepan, melt the butter. Add the leek and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Stir in the oats and cook for 1 minute. Add 1 cup of the stock and simmer over moderate heat, stirring frequently, until nearly absorbed. Continue cooking the oats, adding 1 cup of stock at a time and cooking until the liquid is nearly absorbed between additions. The risotto is done when the oats are chewy-tender and suspended in a thick sauce, about 25 minutes total. Season with salt and white pepper. Stir in the peas, scallion and 3/4 cup of the cheese and cook until the peas are heated through, about 1 minute. Transfer the risotto to bowls, sprinkle with the remaining cheese and serve.