Start by preparing a baking sheet with parchment paper and set the oven to 425 degrees. Core all the peppers, remove ribs and seeds, and place in a single layer. (about 6)

Peel then slice one onion into ~ 4 thick slices.

Cut the top off one garlic head to expose the garlic and set into aluminum foil.

Season everything with salt, pepper, and drizzle with olive oil.

Pop in the preheated oven for ~ 30 min.

After 30 min. take pan out of oven so you can turn over each item using tongs. At this point there should be a nice char and sounds of crackling goodness.

Pop this into the oven for another 10 min. to finish off.

Then set out to cool for ~ 30 min.

You may cover with plastic wrap which will create a steam and make it somewhat easier to remove the skins later. For me, with or without is about the same effort.

Remove the skins as best you can and pop into a food processor or blender. If you don't have either of these, you could try an immersion blender although I have not done this method so I can't speak to how effective it would be.

During the processing phase, I add about 1-2 tsp. of balsamic vinegar to add some more depth of flavor. You can certainly omit this. And you could add it to the peppers prior to the roasting phase so it reduces and sweetens more.

Here, you could also add fresh basil or any other herbs and spices to tailor this to your flavor palette.