Puree the chopped onion in a food processor with the ginger, garlic, coriander, cumin, 1/2 teaspoon salt and 1/2 cup water.
Heat the vegetable oil in a skillet over medium-high heat. Add the sliced onion and cook until golden, 4 to 5 minutes. Add the spice paste and cook, stirring, until slightly dry, 8 to 10 minutes. Add the chicken and cook until opaque, breaking up the meat. Mix the yogurt with 1/4 cup water, add to the pan and simmer over medium-low heat until the meat is cooked through, 2 to 3 more minutes. Add the peas and cilantro and season with salt.
Meanwhile, heat another skillet over high heat. Brush the pitas with oil, season with salt and toast in the skillet, about 1 minute per side. Divide the chicken mixture among the pitas. Top with more yogurt and cilantro. Garnish with cashews and/or hot sauce, if desired.