Place the onion, jalapeño, garlic, breadcrumbs, egg, mayonnaise, Worcestershire sauce, Dijon mustard, salt, black pepper, and rosemary in a medium mixing bowl and mix thoroughly. Add the ground chicken and stir to combine until everything is evenly distributed. Be careful not to overmix or you could get tough burgers.
½ medium yellow or sweet onion, ½ jalapeño pepper, 4 garlic cloves, ¼ cup plain or Italian breadcrumbs, 1 large egg, 2 tablespoons mayonnaise, 1 tablespoon Worcestershire sauce, 1 tablespoon Dijon mustard, 1 teaspoon salt, ¼ teaspoon black pepper, 2 tablespoons fresh rosemary, 1 pound ground chicken