8 medium shallots, peeled
2 1/2 tablespoons unsalted butter
Salt and freshly ground pepper
8 medium parsnips, peeled and cut into 2-inch lengths
4 large turnips, peeled and quartered
4 medium carrots, cut into 2-inch lengths
2 large leeks, halved lengthwise and cut crosswise into 1-inch pieces
2 1/2 cups chicken stock or canned low-sodium broth
1 thyme sprig
1/2 pound small purple or red-skinned potatoes