In a small bowl, combine the vinegar and onion with 1/2 teaspoon of salt. Let stand for 10 minutes, then whisk in the honey, cilantro, mustard, garlic and 1/4 cup of the olive oil.
Meanwhile, in a small skillet, heat 1 teaspoon of the olive oil over high heat until just smoking. Add the jalapeño and cook, turning, until charred and blistered on all sides, about 2 minutes; let cool slightly. Slip off the charred skin. Discard the stem and cut the jalapeño in half lengthwise. Scrape out the seeds and finely chop the jalapeño. Stir the jalapeño into the vinaigrette and season with salt and pepper.
In a large nonstick skillet, heat the remaining 1/4 cup of olive oil over moderately high heat. Season the halibut fillets with salt and pepper and cook, turning once, until they are opaque throughout, about 8 minutes. Transfer the halibut to a platter. Whisk the vinaigrette, spoon it over the fish and serve with avocado wedges or watercress.