Position racks in the upper and lower thirds of the oven; preheat to 425 degrees F. Combine the grapes, fennel, cubed bread and 1 teaspoon each olive oil, rosemary and salt in a large bowl; toss to coat. Spread on a rimmed baking sheet; roast on the upper oven rack until the grapes start to burst and the bread is golden, about 20 minutes. Set aside.
Meanwhile, slice the pork crosswise into 4 equal pieces. Turn each piece cut-side up and press with your hand to flatten slightly. Transfer to a large bowl and add 1 teaspoon olive oil and the remaining 1/2 teaspoon rosemary; season with salt and pepper and toss.
Heat the remaining 1 teaspoon olive oil in a medium ovenproof skillet over high heat. Add the pork and cook until browned, about 3 minutes per side. Add the butter, sage and shallot to the skillet. Cook until the shallot softens, about 1 minute.
Add the chicken broth to the skillet. Transfer to the bottom oven rack and roast until a thermometer inserted sideways into the pork registers 145 degrees F, about 10 minutes. Serve the pork with the grape-fennel mixture; pour the pan juices over the top.