First, you need to cut your chicken breast in half. To do that, place it on a cutting board and, while holding your knife parallel to the cutting board, start cutting down the length of the side of the breast, keeping your incision as centered as possible. Keep slicing the breast widthwise until you get to the other edge.
It might help to place a FLAT hand on top of the chicken breast and apply just a little bit of pressure on it.
Rub each chicken breast half with olive oil and sprinkle salt, pepper and dried thyme all over it. Transfer to a plate and set to rest in the refrigerator while you work on the tomato sauce.
Coat a large saucepan with a little bit of olive oil and heat over medium high heat. When pan is hot enough, add the onion, celery, salt, pepper and oregano and cook for about 5 minutes, stirring often, until fragrant and onion becomes translucent. Stir in garlic and continue cooking for another minute. Add tomatoes and tomato paste, stir well and bring to the boil then reduce heat and simmer gently for 10-15 minutes, stirring from time to time.
Meanwhile, preaheat your outdoor grill to medium high heat. Place the half chicken breasts on the grill and cook for about 4-5 minutes per side, or until cooked all the way through.
Transfer the chicken breasts to individual oven safe dishes. Top each breast with half the tomato sauce, then sprinkle the cheeses overtop the two dishes.
Place on the higher rack of your oven and set to broil, just until cheeses melt and start to bubble and turn golden brown.
Serve with a side of greens.
*While you're at it, why not throw a few veggies on the grill, like bell peppers, zucchinis, eggplants, radicchio or endives? Simply prep your veggies (slice, cut or half them) drizzle with a little bit of extra virgin olive oil, sprinkle with salt and pepper and place them on the grill next to your chicken.