What probably happened
You mashed the plantains too much, used plantains that were too ripe, or didn't add enough of the cooking liquid back in.
Why it happened
Mangú is made from green plantains — starchy, firm, and low in sugar. Their starch granules are tightly packed. When you boil them, the granules absorb water and swell. Mashing breaks those swollen granules open, releasing starch into the mash. A little released starch is what makes mangú creamy. But over-mashing — or using a food processor instead of a masher or fork — shreds too many granules at once, and the released starch turns into a gluey paste. Riper plantains (yellow instead of green) have already converted some starch to sugar, so they don't hold structure when mashed. And skipping the reserved cooking water means there's nothing to loosen the mash back to a creamy consistency.
How to save it now
If the mangú is already gluey, stir in a splash of hot water or warm milk, a little at a time, mashing gently with a fork. The extra liquid will thin the paste and break up some of the gluey starch network. It won't return to perfect fluffiness, but it'll be closer to creamy than paste. Top generously with sautéed onions — the texture contrast helps.
How to prevent it next time
Use completely green plantains — no yellow spots. Boil in salted water until a fork slides in easily but they don't fall apart. Reserve a cup of the cooking water before draining. Mash by hand with a fork or a traditional tostonera press — never a food processor or blender. Add the reserved cooking water a splash at a time until the mangú is smooth but not runny. Stop mashing the moment it comes together.
Tiny kitchen test
Boil two green plantains. Mash one with a fork, adding cooking water slowly. Blitz the other in a food processor for 30 seconds. The fork-mashed one will be creamy and scoopable. The food-processed one will be a stretchy, gluey block you can almost pick up in one piece.
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