How to Make It
Bring a large pot of water to a boil, and add onions. Boil 30 seconds; drain and immediately plunge into a bowl of ice water. Trim root end of each onion, leaving core intact; squeeze gently from top of each onion to remove tough outer skin. Discard skins, and place peeled blanched onions in a bowl.
Heat butter in a large nonstick skillet over medium-high. Once butter is sizzling, add onions in a single layer. Cover and cook, stirring often, until onions are lightly caramelized, 8 to 10 minutes.
While onions cook, whisk together brown sugar, cumin, kosher salt, 1 teaspoon lime zest, and lime juice in a small bowl; pour into skillet, and toss to coat onions. Cover and cook over medium, stirring occasionally, until mixture is syrupy and onions are tender, 5 to 6 minutes.
Spoon onions and sauce onto a platter; sprinkle with flaky sea salt and remaining 1/2 teaspoon lime zest. Serve immediately.