Toast the cumin seeds in a small skillet over high heat until fragrant, about 30 seconds. Grind the seeds in a spice grinder or with a mortar and pestle.
Heat the olive oil and garlic along with the toasted cumin over medium-high heat and cook until garlic begins to sizzle, about 2 minutes. Put in a non-reactive bowl and stir in lime juice, oregano, salt, pepper, and jalapeno.
Preheat stove top or outdoor grill to high heat. Cut plantains in half lengthwise and brush lightly with olive oil. Place on grill, skin-side down, and cook until soft about 3 to 4 minutes per side. Transfer to a platter and serve with the mojo sauce.
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