Prepare a grill or grill pan to medium-high heat.
Sprinkle the chicken, flank steak and shrimp with salt and pepper. Grill the chicken until cooked through, about 6 minutes per side. Transfer the chicken to a platter and cover with aluminum foil to keep warm.
Grill the flank steak to the desired doneness, about 5 minutes per side for medium. Add it to the platter with the chicken.
Grill the shrimp until just cooked through, about 3 minutes per side. Add them to the platter.
Grill the scallions until brown, about 3 minutes per side. Add them to the platter.
Heat 2 tablespoons of the oil in a large, heavy saucepan over medium-high heat. Add the tomatoes, garlic and onions. Cook until the onions are golden, about 8 minutes. Add the oregano, cumin, bouillon, sugar and chiles. Stir until fragrant, about 2 minutes. Add 1 cup water to the saucepan and bring to a boil. Reduce the heat to medium low and simmer until the chiles are soft, 2 minutes. Let the mixture cool slightly. Transfer the mixture to a blender, add 1 cup water and blend until smooth. Strain the sauce into a medium bowl.
Heat the remaining 1 tablespoon oil in the same saucepan over medium heat. Return the sauce to the pan and simmer until slightly thickened, about 2 minutes. Season the sauce with salt and pepper.
Preheat the oven to 350 degrees F.
Place 2 molcajetes (see Cook's Note) in the oven and heat them for 10 minutes (or use 2 small to medium cast-iron skillets). Using oven mitts, remove them from the oven and carefully transfer them to a heatproof surface (be careful, as the molcajetes get extremely hot).
Bring the sauce to a boil. Cut the chicken and flank steak into 3/4-inch-wide strips. Arrange the chicken and steak strips, shrimp, scallions and cheese slices around the edges of the heated molcajetes. Pour the boiling sauce into the center. Serve very hot, with tortillas to make tacos.
A molcajete is a type of mortar made of lava rock, typically used with a pestle (also made of lava rock) to make fresh salsas. The natural rock has an excellent ability to preserve heat and keep a dish hot for a long time.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.
Adapted from "Fresh Mexico: 100 Simple Recipes for True Mexican Flavor" by Marcela Valladolid © Clarkson Potter 2009. Provided courtesy of Marcela Valladolid. All rights reserved.