In a large skillet, warm the oil over medium heat. Add the onion and cook for 5 to 7 minutes, until soft. Add the garlic and mushrooms then cook until the mushrooms have softened and the excess moisture has evaporated. Add leek and cook for another minute then remove from heat and set aside.In a food processor, lightly pulse 3/4 cup of the lentils for about 5 to seconds. Transfer to a large bowl along with cooked vegetables and remaining ingredients. Stir to combine. Taste test and add more salt & pepper, to taste. Place the bowl in the refrigerator for at least 15 minutes, or up to overnight covered.Using your hands, scoop out about 1/2 cup of the mixture and shape into patties. I made about 6 patties while Nina states the recipe should make 4. This will depend on how big you like to make your burgers. Warm a little more oil in the skillet over medium heat then cook the patties for about 5 minutes on each side. Serve warm on toasted buns with vegan mayo, pink kraut and curry ketchup.