Heat 1 tbsp (15 mL) oil in a medium, nonstick skillet over medium heat. Add onion and garlic. Cook, stirring often, until tender, about 5 min. Cool slightly.
In a food processor, combine onion mixture, beans, salt and pepper. Pulse until mixture is finely chopped and combined but not smooth. Transfer to a bowl. Add breadcrumbs and egg; mix until well combined. Form into 4 equal-size patties.
Heat remaining oil in a grill pan or foil-lined grill over medium heat. Grill patties until golden and firm, about 6 min. per side. Serve patties on grilled whole-wheat pitas and top with tzatziki. Garnish with tomato, onion and cucumber (if using).