The day before serving the chicken, in a small mixing bowl, use a fork to mash together butter, zest of 1 lemon, 4-6 cloves garlic, 1 tablespoon chopped thyme, 1/4 teaspoon salt, and 1/4 teaspoon pepper. With fingers, carefully loosen the skin around the chicken breast, leg, and thigh. Spread butter mixture evenly under the skin on breast and upper leg meat.
Squeeze the juice of 1/2 a lemon into the chicken cavity, then season the cavity well with salt and pepper. Stuff the cavity of the chicken with the smashed garlic, 1 of the lemons cut into wedges, bay leaves, and 10 thyme sprigs.
Place chicken on a rimmed baking sheet or plate and squeeze the juice of the remaining 1/2 lemon over the chicken and sprinkle lightly with salt and pepper. Cover and refrigerate overnight.
On the day of serving, remove chicken from fridge and set on the counter for 30 minutes. Position rack in bottom third of oven and preheat oven 450 degrees.
Using olive oil, lightly oil a 9X13 inch baking/roasting dish and place the slice of sourdough in the center of the dish, then place chicken on top of the bread. Drizzle the chicken generously with olive oil, allowing about 2 tablespoons of extra oil to accumulate in the bottom of the pan.
Place dish in preheated oven. Baste every 12-15 minutes, with pan drippings, about roast 40-50 minutes.
Prepare glaze while chicken roasts; mix together until well combined.
As soon as the chicken is almost cooked through (the skin on the drumstick just begins to split or it becomes easy to wiggle), slather on a layer of the creme fraiche glaze using a pastry brush. Continue to roast for 5 minutes.
Slather on another layer of the glaze and roast for another 5 minutes. If the wings or legs are browning too quickly, cover loosely with a small piece of aluminum foil.
Apply one last layer of the glaze and place under the broiler, watching closely, until the glaze caramelizes. At this point, the chicken should be cooked through and nicely browned.
Let the chicken rest for 10 minutes then carve. Cut the sourdough into 4 pieces and serve with the chicken.