In a large saucepan, cover the potatoes with water, add 1 tablespoon of salt and bring to a boil. Simmer over moderate heat until tender, about 20 minutes. Drain and let cool. Quarter the potatoes and put them in a large bowl.
Meanwhile, light a grill or preheat the broiler. Grill or broil the poblanos until charred all over. Transfer to a bowl, cover with plastic wrap and let steam for 5 minutes. Discard the charred skin, stems and seeds and cut the chiles into thick rounds.
Brush the onion slices with 1 tablespoon of the oil and grill or broil them until tender and lightly caramelized on both sides, about 10 minutes.
In a medium skillet, toast the oregano leaves over low heat, until just fragrant, about 10 seconds. Transfer the oregano to a plate. Add the coriander and cumin seeds to the skillet and cook, stirring, until fragrant, about 1 minute. Mince the seeds with a knife or grind in a mortar. Add the bacon to the skillet and cook over moderate heat until crisp, about 6 minutes. Let cool, then coarsely chop the bacon.
In a small bowl, stir the remaining 1/3 cup of olive oil with the vinegar, cilantro, oregano and toasted spices. Add the jalapeños, bacon, grilled onions and dressing to the potatoes and toss gently. Season with salt and pepper.
The salad can stand at room temperature for up to 4 hours.