Bring potatoes and parsnips to the boil in a large pan of water and cook until soft.
Drain and allow to cool slightly before mashing smooth.
Place the mash into a bowl, then add the egg yolk, milk, Nutritional Yeast, salt, pepper and thyme and 1/4 cup of the flour. Use a spoon to mix, then slowly add the remaining flour, kneading with your hands, until less sticky and workable.
Turn the dough onto a work surface sprinkled with flour. Work into a disc, then cut into nine pieces.
Roll each piece into a log, then cut the log into gnocchi-sized chunks. Repeat with remaining dough.
Pan fry gnocchi in hot oil until crisp and golden on all sides.