In a two quart sauce pot, sauté onions and tortilla pieces until they are lightly browned. Add tomato, garlic, and cilantro and sauté until wilted and fragrant. Add stock or broth and ground pistachios and simmer and stir for 20 minutes. Remove from heat and allow to cool long enough to work with. In a blender combine sauce and cleaned poblanos. Puree until smooth and season with salt to taste. Heat and serve in a warm ceramic bowl. Garnish with crema and cotija cheese. Serve with warm thick corn tortillas.