I can't give you an exact recipe, SIB. It's just lots of onion sliced and fried until very dark brown to the point where you fear they'll burn, but they don't. Then the tomato paste (not too much, too much tomato paste is a common moro offender), beans (dried and cooked, my mother will only use canned beans in extreme cases; she makes moro de guandules with frozen rather than canned), part of the bean cooking liquid along with fresh water, to the boil, rice, salt, freshly ground black pepper (though she's stopped thanks to my dad's paranoia that black pepper is spicy).
I've recently gotten her into the tiniest piece of star anise in with the onions. Star anise works to boost umami and its faint sweetness is really nice and something nobody can figure out. She mostly uses it for moro de lentejas, though. We tend to pull it out unless it's a really small piece before the rice gets covered.
She sometimes adds a very small amount of garlic to moro de guandules.
We finish moro with chopped cilantro or thinly sliced scallions.