1. Microwave the sweet potatoes and white potatoes separately on high until easily pierced with a knife. Allow 4 minutes per sweet potato and test; add another minute each until done. Allow 2 minutes per white potato; add another minute each until done.

2. When the potatoes are cool enough to handle, slip the peels off and cut into large chunks.

3. Purée the potatoes in the food processor in two batches, adding 1 cup of water to each batch, until smooth and transfer to a soup pot. Or place the potatos in a soup pot with 2 cups water. Insert an immersion blender into the pot and process until smooth.

4. Bring the mixture to a simmer. Add enough milk to make a fluid but fairly thick consistency. Add the cheese, salt, mustard, and nutmeg. Cover and simmer gently for 10 minutes. Let stand for 1 to 2 hours before serving, or serve at once. Top each serving with a sprinkling of green peas.