Dark green poblano peppers, smaller than a bell pepper but larger than a jalapeño, can be fiery or relatively mild; there’s no way to tell until you taste them. Find them near other fresh peppers at most large supermarkets.
Place four 15-ounce cans and a medium skillet (not nonstick) by the stove. Heat 1 teaspoon canola oil in a large nonstick skillet over medium heat. Place 2 sandwiches in the pan. Place the medium skillet on top of the sandwiches, then weight it down with the cans. Cook the sandwiches until golden on one side, about 2 minutes. Reduce the heat to medium-low, flip the sandwiches, replace the top skillet and cans, and cook until the second side is golden, 1 to 3 minutes more. Repeat with another 1 teaspoon oil and the remaining 2 sandwiches.