Preheat the oven to 350°. On a rimmed baking sheet, toss the tomatoes with 2 tablespoons of the oil and season with salt and pepper. Turn the tomatoes cut side down and scatter the rosemary around; bake on the bottom shelf of the oven for 1 hour and 15 minutes, or until very soft and starting to brown. Let cool, then discard the skins.
Meanwhile, in a roasting pan, rub the chicken all over with 1 tablespoon of the oil and season with salt and pepper. Scatter the onion wedges around the chicken, drizzle with the remaining 1 tablespoon of oil and season with salt and pepper. Roast the chicken and onions in the upper third of the oven for 1 hour and 10 minutes.
Increase the oven temperature to 450°. Add the wine to the pan and roast the chicken for about 20 minutes longer, until the onions are well browned and the chicken is golden and the cavity juices run clear. Pour the cavity juices into the pan. Transfer the chicken and onions to a platter; let the chicken rest for 10 minutes.
Set the roasting pan over moderately high heat and add 4 of the tomato halves and 1/2 cup of water. Simmer, scraping up any browned bits and mashing the tomatoes, until reduced by one-third. Strain the jus into a saucepan and season with salt and pepper. Carve the chicken and serve with the tomatoes, onions, tomato jus and Creamy Cheese Grits.