Make deep gashes in the chicken. Place onions, ginger, garlic and jalapeno in blender. Puree. Transfer to a bowl. Mix in the yogurt, tamarind paste, cumin, salt, turmeric and garam masala. Marinate chicken in this mixture for 2 hours. Saute chicken (reserve marinade) in oil for 5 minutes per side until well browned. Add in reserved marinade. Cover and cook for 25 minutes. Serve over rice.