Coat a 3-quart rectangular baking dish with cooking spray; set aside. In a large Dutch oven bring broth to boiling. Add parsnips, celeriac, rutabaga, turnip, and carrots. Return to boiling; reduce heat. Simmer, covered, about 10 minutes or until vegetables are tender. Drain, reserving vegetables and broth separately.
In the same Dutch oven heat the 1/4 cup butter over medium heat until melted. Add onions; cook about 10 minutes or until onions are tender and golden, stirring occasionally. Stir in garlic; cook and stir for 1 minute. Stir in the 1/2 cup flour; cook and stir for 3 minutes more. Gradually whisk in 4 1/2 cups of the reserved broth, the whipping cream, and sherry. Cook about 10 minutes or until mixture is slightly thickened and reduced to about 3 1/2 cups, whisking frequently. Stir in the reserved vegetables, parsley, thyme, and nutmeg. Cover vegetable mixture and keep warm.
Preheat oven to 400 degrees F. For biscuit topper, in a medium bowl stir together the 2 1/4 cups flour, baking powder, and salt. Using a pastry blender, cut in the 6 tablespoons butter until mixture resembles coarse crumbs. Stir in cheese. Make a well in the center of flour mixture; add buttermilk all at once. Using a fork, stir just until moistened.
Transfer hot vegetable mixture to the prepared baking dish. Drop biscuit topper in 9 or 10 mounds on top of hot vegetable mixture. Bake, uncovered, for 25 to 30 minutes or until filling is bubbly and a wooden toothpick inserted in the centers of biscuits comes out clean.