Preheat the oven to 220C/425F.

In a medium sauté pan over medium-high heat, warm the 2 Tbs. olive oil. Add the red and yellow onions and sauté, stirring occasionally, until tender and golden, 7 to 10 minutes. Add the garlic and sauté, stirring occasionally, until fragrant, 1 to 2 minutes more. Add the vinegar and cook, stirring often, until the liquid is nearly evaporated, 3 to 5 minutes more. Season with salt.

Use the Jamie Oliver method for creating an "oven bag": "Using wide tin foil, make your bag by placing 2 pieces on top of each other (about as big as 2 shoeboxes in length), folding 3 sides in and leaving 1 side open."

Season the chicken breasts with salt and pepper, and place them side-by-side in the foil bag. Sprinkle in the tomato, butter, capers, and carmelized onions. Carefully pour in the wine and seal the foil bag tightly. Slide the foil package onto a roasting tray. Jamie recommends placing the tray on a burner on your stove over high heat for a minute to get the package going. Then put the tray in the oven and bake for 25 minutes.

When it comes out of the oven, open the foil bag, place one chicken breast for each person (use the third to make a great sandwich the next day!) on a plate and spoon over the juices.