Preheat oven to 375°F.
In a skillet over medium heat, melt butter then add onions and season with salt and pepper. Cook until soft and light caramel in color, 20 minutes. Add stock, maple syrup and mustard, and simmer another 15 minutes while bacon and burgers cook.
Bake bacon on a slotted pan or a rack set on a baking sheet until crisp, 18 minutes.
Combine meat with salt, pepper, sage and Worcestershire, and form 6 patties that are thinner at the center and thicker at the edges for even cooking. Heat a cast-iron skillet over medium-high heat with oil, 1 turn of the pan. Cook patties 12-15 minutes turning occasionally.
Serve burgers on pickle-lined buns topped with bacon, blue cheese and maple onions.