Onions have been used over time in medicine and art, but are best known in cuisine, as in four-onion ginger soup.
In a large high-sided skillet, cook bacon over medium heat, turning occasionally, until crisp, about 10 minutes. Transfer bacon to a paper-towel-lined plate to drain. Pour off all but 1 1/2 tablespoons fat; reserve for another use.
Add onions and ginger to skillet; cook over medium heat, stirring occasionally, 30 minutes. Add shallots and sage. Continue cooking, stirring occasionally as onions reduce, until they are very soft and caramelized, about 1 hour. (Add a few tablespoons stock or water if onions start to stick to skillet.)
Preheat oven to 350 degrees. Pour stock into skillet, and bring to a boil. Reduce heat to a simmer, and cook 15 minutes more. Season with salt and pepper.
Meanwhile, cut each bread half diagonally into six 1/2-inch-thick pieces. Brush with oil; season with salt and pepper. Arrange on a baking sheet, and toast in oven until golden, about 20 minutes.
Spread toasts with goat cheese; top with a bacon slice. Divide soup among six bowls; garnish with sage leaves, and serve each with two toasts on the side.
You will need to make the stock at least one day before you make the soup.