Take half the tomatoes and about a third of the onion and place in an oven proof dish with the jalapeños or serranos, the garlic, and half the thyme sprigs. Toss in just enough olive oil to coat without drenching.
Roast in a hot 400° oven until the tomatoes and onions are cooked and starting to caramelize. At the same time, roast the poblanos on a griddle or in the oven until they start to blister a little and soften. Remove chiles from heat and let cool while you make the filling.
Grind the pepitas in a food processor until it becomes a coarse meal.
Heat the palm or coconut oil in a sautee pan and add the cumin, chile powder, and coriander, and toast the seasonings for a minute until they become aromatic without burning. Then add the rest of the onion, the corn, and remaining thyme sprigs, and sautée until it is cooked and the onions are translucent. You may also roast the corn on the cob prior to shucking, for a more fire roasted flavor in the filling.
Add the cooked corn mixture, garlic powder, nutritional yeast, half the cilantro, and about 2/3 of the beans to the pepitas and pulse the processor until it turns into a thick filling.
Remove from the processor, and add the rest of the beans, mixing by hand to incorporate. Season with salt and pepper to your desired taste.
By now the tomatoes and onion mixture in the oven should be roasted and starting to caramelize. Remove from oven, place in a blender, add the remaining cilantro, lemon juice, rice vinegar, and blend (be careful when blending hot liquid it tends to cause combustion in the blender, so pulse gently at first). Season with salt and pepper for taste. Pour a little bit of the sauce back into the baking dish just to coat the bottom.
Now, cut the tops from the top of your chiles, as close to the top as possible while creating a manageable opening. Remove any seeds from the cap and the insides of the chile or they will be a lot spicier than you may be ready for. Fill each chile equally with the stuffing, return caps, and place in the baking dish, then cover the stuffed chiles with some more of the sauce.
Save some of the sauce for your finished plate, about half of the sauce should be reserved. Bake in the oven, covered for about 30 minutes at 375°, then uncover and return to oven and bake for another 15-30 minutes, until chiles are soft and cooked through.
Serve with reserved sauce and garnish with fresh cilantro. Garnish with sprinkles of roasted whole or ground pepitas, if desired.