Preheat the oven to 500°F (260°C). Really – I’m not kidding. In a large roasting pan, combine the sliced onions, baby carrots, celery and about half of the olive oil. Sprinkle with about half of the thyme and rosemary and toss together to mix. Onto this bed of veggies, place the Cornish hen halves, skin-side-up. Brush with the remaining olive oil, and sprinkle with the remaining thyme, rosemary, and the salt and pepper. Set aside until about 40 minutes before you’ll want to eat. (If you’re serving an appetizer and soup, begin roasting just as you sit down to dinner. And take your time.)
Place the pan on the middle shelf of the preheated oven and roast for 15 minutes, undisturbed. Pour in the wine or broth, stir the vegetables around a bit and let it roast until the vegetables are cooked and the Cornish hens are nicely browned – about 15 to 20 minutes longer.