Start by soaking your pasilla and ancho chiles in hot water. Cover and let them steep for 15 minutes.
Char the tomatoes (leave them whole) and onion in the oven under the broiler until they are singed black. I set my broiler to 500℉ and set the rack so that my veggie would be about 2 inches away from the element. Check them at about 5 minutes and give them a turn. Continue to check the vegetables and rotate every few minutes. They don't need to be totally blackened all the way around but you do want your vegetables to have some char which will add a lot of flavor to your pipián! Feel free to do this on a charcoal or gas grill!
Toast your sesame seeds in a skillet over medium-low heat until lightly golden brown, about 5 minutes. Keep your eye on them, stir occasionally and don't let them burn. Once they release their nutty flavor, remove from heat and reserve.
Using the same skillet, toast pumpkin seeds, garlic, cinnamon stick, cumin seeds and oregano over medium- low heat until the pumpkin seeds begin to brown and the spices begin to release their flavors. Remove from the heat and reserve.
By now your chiles should be rehydrated and cool enough to handle. Remove the stems and using the water, remove as many of the seeds as possible. Don't worry if you don't get them all. Reserve the soaking liquid.
Add the rehydrated chiles, tomatoes, onions, sesame seeds, pumpkin seeds, spices and garlic to the blender. Season with salt and pepper. Add 1/2 cup of the chile soaking liquid, straining out the seeds. Blend this mixture until smooth, adding more soaking liquid (or water), if needed.
Give your sauce a taste! Adjust the seasoning to your liking. If you find it on the bitter side, add a few pinches of sugar until it tastes more balanced.
Heat a skillet or sauce pan over medium heat with 1 tablespoon of olive or vegetable oil. Once the pan is hot, carefully pour in your sauce. Cover, reduce heat and let simmer for 15 minutes.
Serve with roast vegetables, chicken, pork or simply enjoy with freshly made tortillas!