Peel parsnips, halve lengthwise, remove and discard woody core, and cut into 2-in. pieces. Put in a large pot. Peel potatoes, cut into 2-in. pieces, and add to the pot. Cover vegetables with cold water and bring to a boil. Add 1 tbsp. salt. Cook until vegetables are tender when pierced by a fork, about 10 minutes. Drain well.
Meanwhile, chop 5 sage leaves. In a small pan over medium-high heat, melt butter. When it stops foaming, add chopped and whole sage leaves. Cook until fragrant, 1 to 2 minutes.
Put vegetables through a ricer into a large bowl, or mash them with a potato masher, a large fork, or an electric mixer. (If using a mixer, be careful not to beat potatoes too much or they will become gluey.) Stir in sage butter and sage, sour cream, and 1/2 tsp. white pepper. Add more salt and pepper to taste. Serve warm.
Note: Nutritional analysis is per 1-cup serving.