Preheat the oven to 425° and position a rack in the lower third of the oven. In a bowl, mix the butter with the minced garlic and Madras curry powder and season with salt and pepper.
Pat the chicken dry. Rub half of the butter under the skin and the rest over the chicken; season with salt and pepper.
Set the chicken breast-side-up on a rack in a roasting pan. Scatter the onion, 4 garlic cloves, tomatoes, chiles and add 1/2 cup of water. Roast for 30 minutes, until the breast is firm and just beginning to brown in spots. Using tongs, turn the chicken breast-down and roast for 20 minutes longer, until the skin is lightly browned.
Using tongs, turn the chicken breast-side-up. Add another 1/2 cup of water. Roast for about 20 minutes longer, until an instant-read thermometer inserted in the inner thigh registers 175° to 180°.
Tilt the chicken to drain the cavity juices into the pan; transfer the bird to a cutting board. Remove the rack from the pan and spoon off the fat. Set the pan over high heat. Add the stock and cook, scraping up any browned bits. Stir in the yogurt just before serving. Carve the chicken and pass the chunky jus at the table.