Wash the fresh spinach under cold running water. Drain, then strip the leaves from the stems and discard the stems and any discolored leaves. In a heavy 2-3 quart saucepan, bring 1/2 cup of water to a boil.
Add the spinach, lower the heat, and simmer tightly covered for about 3 minutes. Drain the spinach in a sieve, cool, and squeeze it completely dry. Chop it as fine as possible. (Frozen spinach needs only to be drained, squeezed dry, then finely chopped.) Set the spinach aside.
Press a small sage leaf on each piece of liver and wrap each piece of liver in a strip of bacon. Thread 5 squares of liver apiece on four 6-8 inch long thin metal skewers, pressing the pieces tightly together.
In a heavy 12 inch skillet, melt 3 Tblsp. of the butter with the oil over moderate heat. When the fat begins to color very lightly, arrange the skewers side by side in the skillet. Fry uncovered, turning the skewers occasionally, for 10-15 minutes, or until the bacon is brown and crisp.
Regulate heat if necessary to prevent the bacon from burning. Meanwhile, in a heavy 8-10 inch skillet, melt the remaining butter over moderate heat.
When the foam subsides, add the onions and garlic and, stirring frequently, cook for 2 to 3 minutes, until they are soft and transparent.
Drop in the spinach, add the salt and a few grindings of pepper, and stir for a few minutes longer, until the ingredients are thoroughly combined and the spinach is heated through.
Spread the spinach on a heated platter and arrange the skewers of liver side by side on top. Then, working quickly, discard the fat remaining in the skillet in which the liver was fried.
Pour in the wine and bring to a boil over high heat, stirring constantly and scraping in the brown particles clinging to the bottom and sides of the pan. Let the sauce boil for a moment or so, then pour it evenly over the liver and serve at once.