In a large bowl, mix together the flour and the salt. Then, make a well in the center and add in the oil and egg. Use a fork to whisk together the eggs and oil and then start combining it into the flour. While stirring, slowly pour in ⅓ cup of water. Continue adding the remaining water as needed until a shaggy dough forms.
2 cups all purpose flour, 1 tsp kosher salt, 1 large egg, ⅓-½ cup warm water, 2 tbsp olive oil
Turn dough out onto a lightly floured surface and knead by hand for about 5 minutes until it's soft and smooth. Wrap the dough in plastic wrap and let it rest at room temperature for at least 30 minutes. This will make it easier to roll out and work with so don't skip it!
While the dough is resting, you can prepare your pierogi filling. In a large bowl, use a fork to mix together the boiling water and the instant mashed potatoes. Set aside to let cool.
1 package Idahoan Four Cheese Mashed Potatoes, 10 oz boiling water
Bring a large pan to medium heat. Once hot, add a bit of oil to coat the bottom of the pan. Then, add diced onion and kosher salt. Saute, stirring occasionally, for 3-5 minutes or until translucent. Remove onions from pan and add them to the bowl with the mashed potatoes.
½ large yellow onion, ½ tbsp olive oil
Next, add the cheddar cheese and season with salt & pepper to taste. Stir with a spatula to combine. Set aside.
2 oz freshly grated sharp cheddar cheese, kosher salt & black pepper
Once the pierogi dough has rested, divide it into two and roll out one of them to ⅛" thin on a lightly floured surface. Keep the other half wrapped in plastic until you're ready to use it so that it doesn't dry out. Use a 3" circle or heart-shaped cookie cutter to cut out the dough. Repeat with the remaining half of the dough - you should get about 40 total, depending how thick/thin your dough was.
If you're making them heart shaped: Add 2 tsp of the pierogi filling to the middle of the dough heart and press down slightly to spread it out in a heart shape. Then, place another dough heart on top and cinch the edges closed with your fingers (or a fork). You will have leftover filling if you're making hearts, since it requires double the dough.
If you're making traditional perogies: Add 2 tsp of the pierogi filling to the middle of the dough circle and press down slightly. Then fold it in half and cinch the edges closed with your fingers (or a fork). You will most likely use up all of the filling, and you may want to make a double batch of the marry me sauce.
Bring a large pot of salted water to a boil. Once boiling, gently add some of the pierogis (I recommend cooking them in batches to prevent sticking). Cook them for about 4-5 minutes total - they should be floating to the top. Remove them from the pot with a slotted spoon and place them on a paper towel lined plate.
At this point you can add the perogies to the sauce, or pan fry them to add a crispy texture (I prefer this). To pan fry them, bring a large pan to medium heat. Then, add 1 tbsp of butter. Once melted, add perogies and cook until golden on each side, about 3 minutes per side (you may have to do this in two batches, adding more butter as needed).
2 tbsp butter