Bring a pot of salted water to a bare simmer. Combine the flour, 2 teaspoons salt, a pinch of pepper and the nutmeg in a large bowl. Whisk the sour cream, seltzer, eggs, herbs and mustard in another bowl, then stir into the flour mixture.
Fill a large bowl with ice water. Working over the simmering water, place about 1 cup dough in a colander and push the dough through with a rubber spatula. Cook the spaetzle for 1 minute after they float to the surface, then remove with a slotted spoon and transfer to the ice water. Repeat with the remaining dough, letting the water return to a bare simmer between batches. Drain the spaetzle, spread out on a baking sheet and pat dry with paper towels. Toss with 2 teaspoons olive oil.
Heat 2 tablespoons butter and 1 tablespoon olive oil in a large skillet over medium-high heat. Increase the heat to high and add half of the spaetzle in an even layer. Cook, undisturbed, until golden brown on the bottom, about 5 minutes. Shake the pan and loosen the spaetzle with a spatula; continue cooking until heated through, about 1 more minute. Season with salt and pepper. Repeat with the remaining butter, oil and spaetzle.