To make the dough. In a medium bowl, combine the flour, salt, butter, yogurt, and egg. Mix until combined. Knead the dough for 2-3 minutes. Cover and let sit 30 minutes.To make the filling. Bring the potatoes to a boil in a large pot of cold water. Salt the water and cook until the potatoes are tender, about 20 to 30 minutes. During the last 10 minutes, add the broccoliDrain the potatoes and broccoli. Pull the broccoli out and set on a cutting board. Return the potatoes to the pot and mash over low heat, adding the butter, cheddar cheese, cream cheese, thyme, garlic, and onion powder. Chop the broccoli and mix in with the potatoes. Season to taste with salt and pepper. Roll the dough out onto a floured surface to 1/8 inch thickness. Using a biscuit cutter, cut out 3-inch circles. Spoon 2 teaspoons of filling into the center of each round. Brush the edges with water and fold half of the dough over the filling to enclose it. Press down the edges to seal, pressing out all the air. Be sure to keep the dough covered as you work to prevent from drying out. At this point, if desired, the pierogi can be flash-frozen on a baking sheet for 30 minutes, then transferred to a freezer bag and frozen for up to 3 months. When ready to cook, bring a large pot of salted water to a boil. Boil the pierogi in batches for 2-3 minutes, or until they float. Drain.To make the butter sauce, melt 1 tablespoon butter in a large skillet. Add the onions and cook 5 minutes, until softened. Season with salt and pepper and continue cooking another 5-8 minutes, until the onions are golden. Add the remaining butter and thyme and cook 2-3 minutes, until the butter is lightly browned. Remove from the heat.Drop the pierogi into the sauce, gently tossing to combine. Spoon the pierogi and onions onto plates. Top with cheddar and thyme. EAT!