1. Preheat broiler. To make the dough, place the ingredients in a food processor. Process until it forms a ball. Knead by hand for 5 minutes, then wrap it in plastic and let the dough rest for 30 minutes.
2. To make the sauce, spray pepper, onion, and tomato with nonstick spray. Place under broiler and cook, turning occasionally until they begin to char (10-15 minutes). Place the veggies, along with the tomatoes, into your food processor or food mill and puree. Force the puree through a sieve to smooth it out a bit. Add salt and pepper, to taste along with some chili paste. Put the sauce in a pan and keep it warm; the vareniki will finish cooking in it.
3. To make the filling, begin to saute onion and garlic in a little vegetable stock. Add mushrooms to the onion mix. Cook this over medium heat until the mushrooms just begin to brown. Add thyme. Take off the heat. Microwave potato for a few minutes until it's cooked enough to mash. Mash the potato in with the mushroom mix. Season, to taste.
4. To assemble, divide the dough into thirds. Roll the dough to 1/8" thick and cut into 3 1/2" circles. Place 1 teaspoon of the filling in the center of each circle. Fold the dough over to form a half-moon shape and seal it shut with your fingers. Place them on a lightly floured towel while you finish making the others.
5. To cook, bring a pot of water to a boil. Add 8 or 9 vareniki to the water. Wait for them to float to the top, then wait 1 minute longer before removing, place into a bowl to keep warm. Cook the remaining vareniki the same way. When all are done cooking, dump them into the sauce and cook over low heat for 5-10 minutes.