Plantain Omelet

When you buy plantains, you have many options. Since a lot of stores sell them green-only, you can wait a few days, until they yellow a bit, and make tostones — chewy, starchy, crisp little pancakes that you cook twice. That’s for another story. 

Or you can wait a week, or two, or three, until they turn black, or nearly so (though I think fully black is …