Heat oil in an 8 inch skillet over medium heat. Add sliced scallions, tomatoes, 1/4 tsp salt, pepper, cumin, saffron, and 2 cloves garlic.
Cook, stirring often, until soft (5 minutes). Add 2 tbsp cilantro, reduce heat to low, and cook, stirring occasionally (20 minutes). Transfer to a small bowl and set aside.
Place remaining salt and garlic, whole scallions and cilantro sprigs, spareribs, brisket, and chicken in a 12 qt pot and cover with water by 1 inch. Bring to a boil, reduce heat to medium-low, and cook until chicken is cooked through (1 hour and 15 minutes).
Remove chicken and cut into 10 serving pieces. Discard skin and carcass. Place chicken on a plate, cover, and set aside.
Add cassava and potatoes to pot and cook for 10 minutes. Add cabbage, cover loosely, and cook until vegetables are tender (20-25 minutes).
Add chicken back to pot, stir in reserved scallion-tomato sauce mixture and season with salt and pepper. Return to just under a rolling boil (5 minutes).
Ladle stew into large serving bowls and sprinkle with remaining minced cilantro. Serve with avocado and rice on the side.
NOTE: To break down cassava for cooking, begin by cutting off the tapered ends to reveal where the cordon begins, then divide the root into manageable lengths, approximately the size of a large russet potato.
Stand the segments up on their flat ends and, using a large chef's knife, cut away the peel of the cassava in strips, rotating the cylinder as you go until it's completely peeled.
Halve the cassava cylinder lengthwise to expose the core. Continue cutting the cassava lengthwise into wedges, making sure to cut through the core.
Using your knife, slice off the inner corner of each wedge to remove the woody flesh and discard. Now that the cordon is removed, it's safe to cook the cassava like any other vegetable, whether you choose to boil, saute, fry, or roast it.