1. Preheat barbecue grill to high for 10 minutes.
2. In a large bowl, combine burger ingredients and mix to combine, being careful not to over mix.
3. Shape into 6 patties, each 1/2-inch (1 cm) thick.
4. Reduce heat to medium and place burgers on lightly greased grill, cover and cook for about 8 minutes on each side or until an instant-read thermometer inserted sideways into the centre reads 165°F (74°C).
5. Meanwhile, in a skillet melt butter over medium heat and sauté mushrooms until golden.
6. Stir in capers, lemon zest and juice and sauté for 5 minutes.
7. Place burger patties on buns.
8. Top with lettuce, tomato, and mushroom mixture.
9. Serve with tzatziki.