FLATBREAD Dough: Mix water and yeast in large bowl; let stand 5 min. Add flour; mix until blended. Place in large clean bowl; cover. Let rise in warm place 20 to 30 min. or until doubled in volume.
PUNCH down dough; sprinkle with salt. Add sour cream; knead 3 to 4 min. or until blended. Cover. Refrigerate overnight. Scale into 20 (3-oz.) balls (or into 5 balls for trial recipe). Refrigerate, covered, until ready to use.
SEASONED Butter: Heat butter and seasoning in saucepot on low heat until butter is melted. (Do not boil.)
FOR each flatbread (3 servings): Heat grill to 400ºF. Roll out 1 ball of dough to 14x6-inch oval; dock well with fork. Brush with 3/4 Tbsp. Seasoned Butter. Place, buttered-side-down, on hot grill. Cook until bubbly and lightly browned. Brush with additional 3/4 Tbsp. Seasoned Butter; turn. Grill until lightly charred. Remove from grill.
BRUSH flatbread with 1/4 cup barbecue sauce. Sprinkle with 2-1/2 Tbsp. onions and 1 oz. corn. Top with 4 oz. pork and 2 oz. cheese. Return to grill; cook until bread is charred and cheese is melted. Cut into 6 slices.