How to Make It
In a microwave-safe bowl, combine the tamarind and water and microwave on high power for 2 minutes. Cover and let stand until the tamarind is softened, about 30 minutes.
Meanwhile, preheat a small skillet. Add the garlic and cook over moderate heat, turning occasionally, until the cloves are blackened in spots, about 8 minutes. Peel the garlic.
Mash the tamarind to a pulp. Strain the puree through a fine sieve into a blender. Discard the seeds and fibers. Add the roasted garlic and the scallions, thyme, chile powder and olive oil to the blender and puree until smooth. With the machine on, add the Coca-Cola in a thin stream until blended. Season generously with salt and pepper. Transfer the mixture to a shallow baking dish. Add the steak and turn to coat. Cover and refrigerate overnight.
Preheat a grill pan. In a large bowl, combine the oil, garlic and serrano chile. Add the sliced vegetables and a generous pinch each of salt and pepper and toss to coat. Grill the vegetables over high heat, turning occasionally, until tender and lightly charred, about 8 minutes. Transfer the vegetables to a cutting board and coarsely chop them.
Brush the chicken with some of the remaining oil from the vegetable bowl and season with salt and pepper. Grill the chicken over high heat, turning once until cooked through and lightly charred, about 8 minutes. Let rest for 10 minutes, then cut into thin strips.
Scrape most of the marinade off the steaks. Grill the steaks over high heat, turning occasionally, until charred in spots, about 10 minutes for medium-rare. Transfer the steaks to a cutting board and let rest for 10 minutes, then thinly slice across the grain.
Mound the steak, chicken and vegetables in 3 bowls and serve with the tortillas, Salsa Verde, Pico de Gallo, cilantro and sour cream.