In a large skillet, melt 6 tablespoons butter over medium heat. Add the onions and cook, stirring, until softened and browned, about 15 minutes. Transfer 1/2 cup to a bowl. Transfer the remaining onions and the wine to a saucepan; reserve the skillet. Simmer the onion mixture over medium heat, stirring, for about 10 minutes. Remove from the heat and stir in the sugar; season with salt.
In the reserved skillet, melt 2 tablespoons butter over medium heat. Add the livers and cook until firm, about 5 minutes. Stir in the vinegar. Transfer to a food processor, add the reserved 1/2 cup onions and pulse into a coarse puree; season with salt.
Arrange 12 wonton wrappers on a work surface and top each with 1 tablespoon of the liver mixture. Moisten the edges of a filled wrapper with water and cover with a second wrapper, pressing the edges to seal. Transfer to a wax-paper lined baking sheet; cover with damp paper towels. Repeat with the remaining wrappers and filling.
In a medium skillet, melt the remaining 4 tablespoons butter over medium heat. Add the sage and cook for 5 minutes; season with salt.
In a large pot of boiling, salted water, cook the ravioli in 3 batches until tender, about 5 minutes. Using a slotted spoon, transfer the ravioli to a large, buttered bowl, topping each batch with the sage butter. Serve dolloped with the onion marmalade.