kosher salt, plus more for seasoning • russet potatoes, peeled and quartered (about 3 large) • unsalted butter • whole milk • sour cream • packed grated Parmigiano-Reggiano cheese or vegetarian Parmesan (optional) • Salt and freshly ground white pepper, to taste • Nutmeg, to taste • bread crumbs (optional) • French lentils, washed • vegetable broth • thyme •