Slice Tribeca Oven Focaccia in half lengthwise and again into sixths, making six sandwiches.
In a medium sauce pan, heat oil over medium heat. Sauté onions, stirring occasionally, until golden-brown, about 10 minutes.
Sprinkle sugar over the onions and cook, without stirring, until sugar melts. Increase heat to high and cook, without stirring, until an golden-brown caramel forms, about 6 minutes. Quickly stir in vinegar and salt and simmer over low heat, until jam has thickened, about 5 minutes.
Layer 1 tablespoon of jam, 2 slices of bacon, lettuce and tomatoes on each bottom slice. Spread 1 tablespoon mayonnaise on each top slice, and stack. Cut each sandwich on a diagonal and serve.