In a very large skillet, cook the spinach over high heat just until wilted, about 1 minute. Drain and press out all of the water. Coarsely chop the spinach. Rinse out and dry the pan.
In a large bowl, knead the chopped bread with the milk, egg, and cheese to form a paste. Knead in the pine nuts, onion, sausage, and spinach, and season lightly with salt and pepper. Using lightly moistened hands, divide the mixture among the pepper halves and lightly pack it in.
In the skillet, heat the olive oil until shimmering. Add the stuffed peppers, filling side down, and cook over high heat until well-browned, about 4 minutes. Turn the peppers and cook until the skins are browned and blistered, about 4 minutes longer.
Add the tomato sauce and chicken broth, cover, and simmer until the sausage filling is cooked through and the peppers are tender, 5 to 6 minutes. Transfer to plates and serve right away.