Cut the beef into bite-sized cubes, trimming any large pieces of fat as well. Season all sides with a little salt and pepper.
In a large pot or dutch oven heat the oil over high heat. Sear the top and bottom (two sides) of all pieces for 2 minutes on each side.
Reduce the heat to low and add the broth, sofrito base, sazon spice, adobo spice and bay leaves. Reduce to a simmer, cover and cook for one hour.
Meanwhile slice up the platano and potatoes so they are a similar thickness (and will cook within a similar amount of time). Cut the corn into small rounds.
Remove the bay leaves from the soup. Add the vegetables and oregano.
Cook covered for another 30-45 minutes, until the vegetables are tender.
Sprinkle the chopped cilantro over the top and stir into the soup before serving.
Notes
Chuck roast can be substituted for london broil or any cut of beef you might use in a stew
Depending on your size of pot you may need to sear the meat in a couple of batches.
The vegetables (platano, potatoes and corn) can be substituted for other root vegetables. I think sweet potatoes sometimes add a fun (sweet) flavor to sancocho.
If after simmering for an hour you find the liquid in the soup seems to have reduced more than you would like you can add a little water or more broth.
I like to serve this alongside steamed white rice and lime wedges.