Preheat the oven to 350 degrees F.
Cut the focaccia into small cubes and spread them out on a cookie sheet. Bake until dry and crunchy, 20 to 30 minutes.
In a thick-bottomed pot, heat the butter and oil over medium low. Add the onions, celery and thyme; sweat the vegetables for 10 to 12 minutes. Season with salt and pepper.
Turn the heat up to medium high and add the apples. Cook until the apples start to brown, 8 minutes.
Turn off the heat and add the chestnuts and bread; toss to mix.
Pour in the chicken stock, 1 cup at a time, mixing well between additions -- you want the mixture moist, not soaking wet.
Transfer the stuffing to a casserole dish, cover with foil and bake for 45 minutes. Uncover and bake for another 15 to 20 minutes, until golden.