Trim the leeks, leaving about 1 inch of the tender green tops intact and cutting them all to the same length. If the leeks are small, leave them whole; if medium sized, cut them in half lengthwise.
Using kitchen string, tie the leeks in a bundle, securing it in 2 or 3 places. Bring a large pot half full of water to a boil over high heat. Add the salt and the leeks and boil, uncovered, for 5 minutes. Drain and let cool for a few minutes. When the leeks are cool enough to handle, snip the string and separate the leeks. Set aside.
Bring a small saucepan three-fourths full of water to a boil over high heat. Add the onions and boil, uncovered, for 3 minutes. Drain and plunge the onions into cold water. When the onions are cool, trim off the root ends. Using your fingers, slip off the skins, then cut a shallow cross in the root end of each onion to keep the inner layers from protruding during cooking. Set aside.
Place the leeks in a sauté pan or fry pan large enough to hold them in a single layer. In a small bowl, stir together the 3/4 cup broth, the wine, olive oil and tomato paste until well blended and pour over the leeks. Tuck the bay leaves under the leeks, distributing them evenly in the pan, and scatter the peppercorns over the top. Arrange the onions around and among the leeks, making sure they are immersed in the liquid. The liquid should just barely cover the leeks; add more broth if needed. Place the pan over medium heat and bring to a simmer. Reduce the heat to low and barely simmer, uncovered, until the leeks are tender and the liquid is reduced to a few spoonfuls, about 45 minutes. Remove from the heat and let cool.
To serve, arrange the leeks in a single layer in a serving dish. Arrange the onions and bay leaves over the leeks, then spoon the sauce, including the peppercorns, over the top. Sprinkle with paprika and serve.