Heat the butter in a large nonstick skillet over medium-high heat. Once hot, add the mushrooms, salt, and pepper and cook until the mushrooms are golden, about 5 to 7 minutes, stirring occasionally. Turn the heat down slightly if necessary.
Turn the heat down to low. Add the garlic and thyme and cook 2 minutes more, stirring constantly. Set aside to cool while you make the dough.
For the Dough:
Add the yeast and warm water to a small bowl and stir to combine. Set aside until foamy, about 5 to 10 minutes.
Whisk together the almond flour, psyllium husk powder, and baking powder in a medium bowl and set aside.
Add the part-skim mozzarella and cream cheese to a large microwave-safe bowl. Microwave for 60 seconds and then give it a stir, and continue microwaving in 20-second increments until the cheese is fully melted and combined when stirred.
Stir the yeast mixture into the melted cheese until combined, and then stir in the beaten egg until combined. Stir in the almond flour mixture until it forms a dough.
Oil your hands and knead the dough a couple times until it comes together as a ball. Divide the dough into 8 equal balls. Roll each dough ball out between 2 pieces of parchment paper to a circle about 5 inches in diameter.
Divide the mushroom filling between the 8 dough circles, leaving a 1-inch border. Place 1/2 tablespoon cottage cheese on top of the mushroom filling on each dough circle.
Fold each dough circle over so it looks like a half-moon, lightly pressing the outside together to seal it.
Place the pierogis onto a plastic-wrap lined plate and cover with plastic wrap. Refrigerate at least 12 hours, but up to 3 days.
To Cook the Pierogis and Onion Topping:
When you’re ready to cook the pierogis, preheat the oven to 300F.
Heat 2 tablespoons unsalted butter in a large nonstick skillet over medium-high heat. Once melted, add the pierogis and cook until browned on both sides, about 2 to 3 minutes per side, flipping once.
Once they’re browned, transfer the pierogis to a baking sheet and put in the oven for 5 to 10 minutes while you cook the onions.
To the same skillet that you browned the pierogis in, add the onion and turn the heat to medium. Cook the onion until softened and starting to caramelize a bit, about 5 to 7 minutes, stirring occasionally.
Serve the pierogis warm topped with the onion, along with the sour cream and dill for topping.
Net Carbs: 5g per serving (1 pierogi with toppings)