Add potatoes and enough water to cover by 2” to medium saucepan over medium-high heat. Bring water to boil; season with Adobo. Reduce heat to medium-low. Simmer until potatoes are tender, 15-20 minutes; drain potatoes reserving ½ cup cooking liquid.
Meanwhile, heat oil in large skillet over medium heat. Add onions; cook, stirring occasionally, until soft and caramelized, about 20 minutes; add garlic; cook until fragrant and just golden brown, about 1 minute; keep warm. Set aside ¼ cup olive oil-onion mixture.
Pass potatoes through food mill or potato ricer into saucepan. Or in saucepan, use potato masher to mash potatoes. Set saucepan over low heat. Using wooden spoon, slowly stir in reserved cooking water until smooth and combined. Stir in remaining olive oil mixture until thoroughly incorporated; season potatoes with Adobo. Transfer mashed potatoes to serving bowl; drizzle with reserved olive oil mixture.
For soft and creamy next-day mashed potatoes, try this double-boiler trick: Place leftover mashed potatoes in a heat-resistant bowl set over a pot of simmering water (making sure the bottom of the bowl does not touch the water). Cook, stirring often and adding water in 2 tbsp. measurements to loosen, until potatoes are warmed through and creamy. Serve immediately.