Add ground pork, finely diced leeks, peeled and shredded tart green apple, worcestershire sauce, sage, garlic, salt, and pepper to a mixing bowl. Before dicing your leeks, be sure to clean them well so that you don't get grit in your burgers. To shred the apples I used a box grater.
Using your hands, gently combine the ingredients and then form them into 4 patties. Lightly oil them with 1 tsp olive oil (if your grill grates are getting old, you may need to use a little more).
If your pork hasn't come to room temperature yet, allow it time to do so before beginning to grill, so that they cook more evenly.
Grill the patties over direct medium heat (350ºF - 450ºF) until they're cooked through, about 12-15 minutes, turning once. Keep the lid closed as much as possible while grilling.
While the burgers are grilling bring your glaze ingredients to a simmer in a saucepan. Simmer until the contents are reduced by half and have become a syrupy glaze. Set aside until the burgers are done cooking.
Cut a sour green apple width-wise and remove the seeds to make an apple ring. Slice 4 pieces of gruyere cheese. Set aside.
Once you've removed the burgers from the grill, coat them with your maple apple glaze using a pastry brush. At this time you can toast your brioche buns if desired.
To assemble the burgers, top the patty with an apple ring and a slice of gruyere cheese, and drizzle on a little more maple apple glaze for good measure. Top off with the brioche bun and take a big bite!