If you can't choose between this and the spicy pizza version, just make them both. A piroshki party is a very cool thing. The fillings are no-cook and the dough doesn't require a second rise after shaping, so there's really no excuse. Aggressively wringing out moisture from the frozen spinach will ensure your piroshki are melty-cheesy, not sad-soggy.
Whisk together 1 large egg, room temperature, â cup (150 g) sour cream, 2 tsp. instant yeast, 2 tsp. sugar, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt in the bowl of a stand mixer. Fit mixer with the dough hook. Add 2 cups (250 g) all-purpose flour and mix on medium-low speed until a smooth, slightly tacky dough comes together, 5-8 minutes. Remove bowl from mixer and drizzle dough with 1 tsp. vegetable oil; turn to coat. Cover with a large plate or plastic wrap and let sit in a warm, draft-free spot until dough is doubled in size, about 2 hours.
Mix one 10-oz. package frozen spinach, thawed, squeezed dry, 6 oz. low-moisture mozzarella, coarsely grated (about 1½ cups), 6 scallions, thinly sliced (about 1 cup), ½ cup frozen corn kernels, thawed, drained, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl to combine. Divide dough into 8 equal pieces on a clean surface (don't flour); cover loosely with plastic wrap. Working with 1 piece at a time and keeping remaining pieces covered, press dough into a 5"-diameter round with your fingers. Place 2 Tbsp. of the filling in the center of the dough, then fold dough over filling, enclosing completely. Press edges together to seal.
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