In a large bowl, mix together the curry powder, garam masala, chicken bouillon, lemon juice, 1/2 teaspoon salt and 2 tablespoons olive oil. Rub all over the chicken and let marinate for 30 minutes.
Preheat the oven to 400 degrees F. Line a baking sheet with foil.
Spread the chicken out onto the lined baking sheet and bake until cooked through, about 45 minutes. Let cool slightly, then slice into bite-size pieces.
Thinly slice three-quarters of the onion and dice the remaining one-quarter. Set the diced onion aside for the topping. Heat the remaining 2 tablespoons olive oil (or 2 tablespoons of the marinade) in a skillet over medium high and add the sliced onion. Cook, stirring occasionally, until browned, about 7 minutes.
Add the chicken and some of the marinade to the onions and cook until crisp and beginning to brown on the edges, about 5 minutes. Season with salt and pepper.
Warm the tortillas on the stove or in the microwave. To assemble, fill them with chicken, Tahini Sauce, diced onion and cilantro.
In a small bowl, mix together the tahini, lemon juice and 3 tablespoons cold water until the mixture thickens. Season with salt and pepper.