Heat olive oil in a braiser over medium-high heat. Add sausage and cook, without moving, until golden brown, 4 to 5 minutes. Flip and brown on other side, another 4 to 5 minutes. Carefully remove sausage and add the fennel to the pan, and season with salt and pepper. Let cook until slightly softened, 2 to 3 minutes, then add the grapes, balsamic and thyme. Cook, stirring occasionally, until fennel is softened and the balsamic is reduced, about 5 minutes. Place the sausage on top of the grape mixture.
Transfer to the middle rack of the oven and roast until sausage registers 160 to 165 degrees F, 15 to 20 minutes. Remove sausage carefully and take out skewers.
Scoop out the inside of the bottom of the boule. Spread some tapenade in the hollow, then fill with the fennel-grape mixture. Place sausage on top and sprinkle the feta over everything. Spread more tapenade on top half of boule and close the sandwich. Cut into wedges and serve immediately, or wrap in butcher paper or a large napkin to enjoy later.