Rinse the onions under cold water and pat them dry. Trim off the root ends and about 4 inches off the tops of the green stems, then discard any wilted pieces. Coat the onions evenly with olive oil or lightly spray with cooking spray.
Heat a comal, cast-iron skillet, or grill pan over medium-high heat. Add the onions and cook until softened, lightly charred, and the bulbs feel tender when pressed, about 3 to 5 minutes per side. Try not to flip them too often so they have time to caramelize and develop color.
Remove from the heat and season with kosher salt. Squeeze fresh lime juice over the top, then add a few dashes of Jugo Maggi seasoning, soy sauce, or Worcestershire, if desired.
Serve immediately with carne asada or other grilled meats. You can serve them whole as a side or chop them up and add them to tacos.
Notes
Finding cambray onions. Cambray onions are commonly sold at Mexican grocery stores and many international markets. If you can’t find them, grab regular green onions, double the amount, and pick the ones with the biggest bulbs.
Knowing when they're done. The bulbs should feel tender when you press them and look blistered and browned. That's when the sweetness has come out.
How to store. Let the onions cool, then keep them in an airtight container in the fridge for up to 3 days. Reheat in a skillet over medium heat or in the microwave for a minute or two.