Drain your green chilies. Slice down one side of each chili to open. Scrape out seeds, rinse, and pat dry. Check the size of your chilies compared to your *baking dish (13"x9") and trim chilies if needed, so you can lay out 6 chilies without touching and leaving a little room in between. Trim chilies as needed. There will be two layers, so you will need to do this with 12 chilies. If you want larger squares you can use a larger dish or a deep cookie sheet, but you'll need to multiply the chickpea flour ingredients (flour, oil, water, spice), by maybe a quarter or a half, depending on how large of a space you are covering.
Grate, crumble, or slice your cheese, making sure it's ready to grab.
Line a 13"x9" baking dish with parchment paper, then spray or rub paper and sides with cooking oil. Make sure enough of the parchment is sticking out from both sides of the dish, to easily lift out contents later.
Take half the chickpea flour, half the coconut oil, half the water, and half the measured amount of spice mix and put them in a medium saucepan. Once all ingredients are in your pan, turn the burner to medium-high and whisk continuously until mixture starts to thicken. Once the mixture thickens, spoon into your lined baking dish and spread evenly with a spatula. Quickly lay six of the chilies on top, leaving space in between. Gently press, and then sprinkle each with some of the cheese. Next lay the other six chilies on top. I alternated directions with the second layer of chilies so that the pointed end of each bottom chili is at the straight end of each top chili so that the two layers together almost make a square shape. After this, I topped them all with a little more cheese.
Working fast repeat the beginning of step 4 with the remaining flour, oil, water, and spice. Once ready spoon on top of your chilies and cheese. Then very carefully smooth out with a spatula, as good as possible. Press down along the edges, to seal the two layers some. This will now need to sit and firm up for 20-30 minutes. The quicker you get the second layer of batter on these, the better they should hold together, so it is important to work fast.
While those sit, make your sauce. Line a baking sheet with aluminum foil and lightly oil. Scatter tomato halves (skin side up), onion slices, garlic, and serranos (skin side up) over the baking sheet, and place under the broiler on high, for about 7 minutes. The tomatoes and chilies should be starting to blacken and blister. Remove from oven. Depending on your spice level preference, and the heat of the chilies, I'd set aside one or two of the chili halves. Then place the rest of your broiled veggies to your blender along with the remaining sauce ingredients. Blend on high until smooth. Taste and add more chilies if desired. I usually like 1 1/2, but sometimes use more or less depending on the individual chilies. Once ready, transfer to a saucepan and set on stove to reheat later.
After your pan of layered batter and chilies have sat and firmed up for about 20-30 minutes, carefully lift out of dish using sides of parchment paper. Now take a knife and cut around each chili, creating individual squares. They'll be covered, but you should have a good idea where each is located.
For cooking squares, you have two options, I actually do both methods at the same time, so I can get more down at once. I preheat my oven to 250° to keep finished chilies warm while the rest are still cooking. If using air fryer, have it preheated to 370°. Lightly spray the bottom and your multi-level rack with cooking oil. Carefully place one square on the bottom and one on the top rack. These will take about 10 minutes, however, my bottom layer always takes a little longer. If using a skillet, heat olive oil, over medium heat, and fry each side for approximately 3 minutes. Be very careful when flipping, so they stay together. If using air fryer, flipping isn't necessary. Keep finished squares warm by placing on a parchment-lined baking sheet in a heated oven.
Once the last of your squares are soon to being done, reheat your sauce. For serving, top each chile relleno square with sauce, a dollop of vegan sour cream, and cilantro. Enjoy!