1) Put a griddle pan over medium-high heat or preheat a gas or charcoal barbecue.
2) In a large bowl, drizzle the chicken, flank steak, and shrimp with olive oil and season on all sides with salt and pepper, to taste. Grill the chicken until cooked through, about six to eight minutes per side. Grill the flank steak to desired doneness, about five minutes per side for medium. Transfer the chicken and steak to a cutting board and let the meat rest for five minutes before slicing. Grill the shrimp until just cooked through, about three minutes per side. Grill the spring onions until brown, about two to three minutes per side.
3) Grill the cheese for ten seconds on each side. Transfer the shrimp, scallions and cheese to a large plate.
4) For the sauce: in a large heavy saucepan, heat two tablespoons of the oil over medium-high heat. Add the onion, tomatoes, and garlic. Cook until the onion is golden and very soft, about eight minutes. Add the chillies, oregano and cumin. Stir until fragrant, about for two minutes. Stir in 225ml water and bring to a boil. Reduce heat to medium-low and simmer until the chillies are soft, about two minutes. Let the mixture cool slightly, about five minutes. Transfer the mixture to a blender or food processor, add the remaining 225ml of water and blend until smooth. Strain the sauce into a medium saucepan and simmer until slightly thickened, about five to eight minutes. Season with salt and pepper, to taste.
5) Cut the chicken and flank steak into 2cm wide strips. Arrange the chicken strips, steak strips, shrimp, scallions, and cheese slices around the edges of a heatproof dish. Pour the warm sauce into the center. Serve very hot, with tortillas to make tacos.
Notes: The sauce can be made one day ahead, covered and refrigerated. Bring to a boil before using.