I've made 3 batches for the freezer so far.
I do the bulk of it on the grill at 400 degrees. You want to achieve good grill marks on all. If the onions are large cut in half, one head garlic
2 pablano chilies, 2 jalapenos and 20 or so tomatoes depending on size and type. just wash stuff off and place on hot grill
burn the chilies and place in plastic container or bag to cool, finish off the onions and garlic cooking on both sides, place into
plastic container with lid let steam, now turn the tomatoes for the first time you want to achieve good grill marks on the tomatoes.
Let the tomatoes finish cooking on other side for another 7 minutes or so then toss into plastic container with lid to finish off.
Clean up the chilies and rough chop, chop onion up, squish the garlic out of their skins. Heat a large pot and put in a good table spoon of butter and a little less olive oil, dump in onions and cook until softer add the chilies and garlic, then as you pull skins off the tomatoes toss into the pot. I let it cook down a while then put into a blender and whiz it up. This is the best sauce ever.
The first batch I made I used three pablanos and three jalapenos (one seeds and all) that was a little too hot for wife but I like it.
The second batch as described above I did not use any seeds or stems in the chilies it's still warm but it's is so good. The last batch I used just 3 pablano's turnned out great.