Joe Zavala Jr., left, and partner Drew Wright work the cutting board at Zavala's Barbecue in downtown Grand Prairie. (Photo ©Tom Fox/Texas BBQ Posse) Newly-opened Zavala’s Barbecue had been on the Posse’s radar for the past couple of weeks, so we headed south to old downtown Grand Prairie last Saturday. The sky was deep blue and it was 70-degrees. It also just happened to be Cinco de Mayo, the making of a perfect BBQ day. Zavala’s had been open on Main Street for eight Saturdays when we visited, growing from a small catering outfit to a pop-up and eventually to…