SPICY CILANTRO LIME COLESLAW: In a small bowl, whisk together mayonnaise, lime juice, rice vinegar, garlic, Sriracha, and sugar until the sugar is dissolved.
In a large bowl, toss together coleslaw, red onion, and cilantro. Pour dressing over the slaw; toss to coat. Cover tightly and chill until ready to serve tacos.
OVEN ROASTED CORNED BEEF BRISKET: Preheat oven to 350 degrees. Rinse corned beef brisket under cold water and pat dry with paper towels.
Place corned beef brisket into a 9x13-inch baking dish, fat side up. Spread mustard evenly over the top of the brisket and sprinkle with the contents of seasoning packet. Tightly cover the baking dish with aluminum foil and place in oven for 2 1/2 hours.
Remove the roasting pan from the oven. Carefully remove the foil and pour the chicken broth around the corned beef. Tightly seal pan with aluminum foil and return to oven for 1½ to 2 hours.
Remove the brisket from the oven and loosely tent with aluminum foil. Let rest 15-20 minutes. Once cool enough to handle, thinly slice for tacos.
TO ASSEMBLE: Top warm tortillas with corned beef, coleslaw, and Pico de Gallo. Garnish with cilantro leaves and a squeeze of lime.