Heat the oven to 325 degrees. Season the chicken with salt and pepper. (If you can do this in advance, please do.) Drizzle it with the olive oil and sprinkle with the fennel seeds. Stuff the cavity with half the oregano and place in a large baking dish.
Scatter the tomatoes, garlic, butter and remaining oregano around the chicken. Roast until the chicken is golden brown and completely cooked through, and the tomatoes are nice and jammy, 2 ½ to 3 hours.
Add the vinegar to the tomatoes and let the chicken rest in the baking dish for 10 minutes. Place toast, if using, on serving platter and spoon the jammy tomatoes over or around the toast. Carve the chicken and place on top of the toast to catch the juices.