Fill a large pot with cold water and the juice of a lemon. Add a generous sprinkle of kosher salt and the plantains. Cook over high heat, and bring to a boil.
Meanwhile, in a small saucepan, add the olive oil and garlic, and cook over medium to low heat. When the garlic reaches a lightly golden color, add the onions, oregano, sour orange juice, and salt and pepper. Lower the heat, and let the sauce simmer until the onions are translucent.
In the large pot with the plantains, prick the bananas with a fork to test their tenderness. When they can easily be pierced, take the pot off the heat.
Reserve 1 cup of the starchy cooking liquid, then drain the plantains.
Return the plantains to the pot, and mash using a potato masher. Thin the mixture with the cooking liquid until it reaches your desired consistency. Mix in some of the mojo sauce, reserving some onions to serve on top.
Serve immediately.
Recipe by The Cuban Reuben at https://thecubanreuben.com/fufu-con-onion-mojo/