Place onions, corn and carrots in bottom of Römertopf or terracotta baker. Add salt and fresh ground pepper to taste and beer.
Remove fatty membrane on back of ribs. Coat ribs with mustard and then sprinkle with rib rub, coating generously. Add ribs to Romertopf. Cover.
Place Römertopf in oven and heat to 400°F. Cook covered for 2-3 hours. Check after 1 1/2 hours and add BBQ sauce. Cook until ribs are no longer pink inside.
For leftovers: Strip meat from bones. Slice potatoes into bite-size pieces. Heat in microwave or pan. Warm tortilla. Place meat, potatoes, carrots, onions and salad into tortilla. Top with a bit of spicy Thais sauce.