Melt butter in saucepan. Whisk flour with cream to dissolve, then add mustard, thyme and pepper. Add liquid and spices to melted butter. Stir until thickened and smooth.
Place potatoes, parsnips and onions in crock pot. Pour cream mixture over top.
Cover. Cook on low for 7 hours or high for 3-4 hours until potatoes are tender. Stir in cheese. Cover and let stand until cheese melts.