Preheat oven 130°C fan forced. Toss the beef in the Mexican seasoning, Heat 1 teaspoon oil in an ovenproof casserole dish over a high heat. Add half the beef. Cook, for about 2-3 minutes, turning until browned all over. Remove to a plate. Repeat with oil and remaining beef.
Reduce heat to medium. Add remaining 2 tsp oil and the onion. Cook, stirring occasionally 3 minutes until soft. Add the stock and tomatoes. Return the beef and any juices, bring to simmer. Press a piece baking paper onto the surface and cover with lid. Transfer to the oven. Cook for 3 hours. Remove from oven, allow to stand.
Preheat oven 220°C fan forced. Scatter the cauliflower, capsicum, carrots, corn (still on the cob) and chickpeas into a large roasting pan. Drizzle with oil and sprinkle with seasoning. Turn to coat. Roast 30 minutes, turning vegetables once or until vegetables are tender. Remove corn to a board, cut off the kernels. Return the kernels to the vegetables. Whisk all the dressing ingredients together.
Remove beef to a board, coarsely shred. Add to the vegetables. Spoon into bowls, spoon over the dressing. Scatter with coriander and serve with guacamole and extra lime wedges.