Preheat the oven to 400 degrees.
Char the red bell peppers and poblano pepper by setting them directly on a gas burner with the flame turned to medium-high and rotating them as they blacken. If the skins are fully charred but the flesh is not yet tender, transfer the peppers to a foil-lined sheet pan and bake until soft to the point of collapsing, 10 to 25 minutes depending on the thickness of the peppers.
Pierce the eggplants a few times with a sharp knife. Set them on a foil-lined sheet pan and bake until soft to the point of collapsing, 30 to 40 minutes.
Transfer the vegetables to a large bowl, cover with a plate or plastic wrap, and let steam and cool.
Using your fingers or a paring knife, peel the eggplants and the peppers. Remove any large seed pockets in the eggplant, and then stem and seed the peppers. Transfer the vegetables to a large bowl. Using your hands, pull the peppers and eggplant apart into chunks, then massage them between your fingers to form a coarse paste. Set aside.
Using a mortar and pestle, the back of a heavy knife and a cutting board, or a mini food processor, crush together the garlic and salt, forming a paste. Add the garlic paste, olive oil, vinegar, black pepper and the red pepper flakes and cayenne, if using, to the eggplant mixture and mix well. Taste and adjust the seasoning with more vinegar, salt, and black, red and cayenne pepper if needed.
Transfer to a jar and top with ¼-inch layer of olive oil. Cover tightly and refrigerate at least overnight before serving. Season lightly once more before serving.