For the chicken: In a large stockpot, sear the chicken in some olive oil over medium-high heat. Then add the garlic, onions, enough water to cover the chicken and some salt and pepper. Simmer until the chicken is fall-off-the-bone tender, about 1 hour. Strain and set the chicken pieces aside.
For the sauce: Using a mortar and pestle or a food processor, reduce the pumpkin seeds, sesame seeds, garlic, guajilla chiles, ancho chile, cloves, onions, cinnamon stick and some salt and pepper into a thick puree. Set aside.
Pour the sauce into a large skillet over low heat. Add the chicken pieces and cook, coating the chicken pieces with the sauce. Add some chicken stock to loosen the sauce if necessary and simmer, about 20 minutes. Garnish with cilantro and serve warm with tortillas.