
In a bowl, combine all the ingredients for the Big Mac-style sauce. Set aside.
Melt the butter with the oil in a large skillet over medium heat. Add the leeks and salt. Cook for 8 to 10 minutes, stirring regularly, until the leeks begin to soften
Add the maple syrup and water, then continue cooking for 10 to 15 minutes over medium-low heat, stirring regularly, until the leeks are very tender, golden and slightly sticky. Set aside
In the same skillet, toast the buns on both sides. Set aside.
Divide the ground beef into 8 small portions of about 60 g (2 oz) each. Loosely shape into balls, without overpacking the meat.
Heat the same skillet used for the buns and leeks over medium-high heat. Using a sturdy spatula, smash the meat to form a very thin patty. Season with salt and pepper to taste.
Cook the patties for about 2 minutes, or until the underside is golden brown and crispy.
Gently flip the patties, then add a slice of cheddar and continue cooking for about 1 to 2 minutes, or until the cheese has melted.
Remove the patties from the skillet. Generously spread Big Mac-style sauce on the bun, add two patties, then the caramelized leeks.
Close the burger with the top bun and serve immediately.

Leeks contain large quantities of vitamins A, B1, B2, B3, B9 (folic acid) and C.
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