In large pot over medium-high heat, bring 6 cups water to boil. Add yuca and 1 ½ tsp. adobo. Cook yuca until completely tender, about 20 minutes. Drain, reserving ½ cup water. Transfer yuca to cutting board. Carefully halve yuca pieces lengthwise; remove and discard woody cores.
In large bowl, using potato masher, mash yuca with reserved water, 2 tbsp.olive oil, 1 tbsp. garlic and remaining adobo until smooth and combined; cover with foil to keep warm.
Meanwhile, heat remaining oil in medium skillet over medium-high heat. Add onions; cook until soft and translucent, about 10 minutes. Add sazón and remaining garlic; cook until fragrant, about 15 seconds more.
To serve, transfer yuca mixture to serving bowl; drizzle with garlic sauce.