Preheat the oven to 400°. Place the fennel seeds in a small skillet over medium heat. Gently shaking pan back and forth over heat, toast seeds until fragrant, about 2 minutes. Transfer seeds to a plate.

Heat 1 teaspoon oil in a medium skillet over medium-high heat. Add the mushrooms and cook, stirring, until almost all of the liquid in the pan has evaporated, 5 to 6 minutes. Remove from heat and season lightly with salt and pepper.

Bring a large saucepan of lightly salted water to boil. Add chard leaves and cook until tender, about 4 minutes; drain and pat dry.

Open the lamb like a book and place on a clean work surface; season with salt and pepper. Sprinkle with the fennel seeds, then cover with chard and mushrooms. Roll up lamb, cover with the pancetta, and secure at 2-inch intervals with kitchen twine; tuck rosemary and sage into twine.

Place the lamb in a roasting pan, drizzle with remaining 2 tablespoons oil, and season with salt and pepper. Roast for 20 minutes. Add ½ cup of the wine and ½ cup water to the pan; baste. Roast for 30 minutes more, basting every 10 minutes, then add the remaining 1⁄4 cup wine and ¼ cup water, and continue to roast, basting once or twice, until an instant-read thermometer inserted into thickest part of lamb reaches 140° to 145° for medium, 15 to 20 minutes more. Transfer the lamb to a cutting board and let rest for 10 minutes. Cut and discard the twine. Slice lamb and serve with pan juices.