How to Make It
In a medium bowl, lightly beat the egg and egg white with 1 cup of water. Stir in the flour, chives, parsley, thyme, and salt just until a sticky dough forms. (The spaetzle dough can be refrigerated, covered, for up to 4 hours.)
Bring a large pot of salted water to a simmer. Pat one-third of the dough into a 3-inch square on a small cutting board with a handle, or on the back of a square cake pan. Using a moistened metal spatula or chef's knife, cut off a 1/4-inch-thick strip of dough and scrape it into the simmering water. Working quickly over the pot, slice the remaining dough into the simmering water, moistening the spatula or knife if it sticks to the dough. Stir the spaetzle and cook over moderate heat until the dumplings float to the surface and are just cooked through, about 2 minutes. Using a slotted spoon, transfer the spaetzle to a bowl and keep warm. Repeat the process with the remaining dough.
Add the Three-Mushroom Gravy to the cooked spaetzle and toss gently to combine. Serve at once.