If you can, just grill them whole, serve them as a side dish to carne asada or whatever, or with tacos...
Or braise them. Cut off the really dark green separated leaves (use that for pesto), keeping the white and light green cores. Slice in half lengthwise. Season the cut side.
In a pan slowly melt some butter. Place the green onion halves cut side down and hold them down in with a spatula while they sizzle and get a nice deep brown caramelization on the cut side. Take your time, that's the most important step.
Once you're happy with the color, add a bit of water to the pan. Cover and braise for a few minutes, until a toothpick or the tip of a good knife goes inside the onions without any resistance whatsoever. It should be literally melting in your mouth.
Serve cut side up.
While they're great like that, you can think of many ways to flavor them further. To the braising liquid, add your choice of ... a bay leaf, cumin seeds, coriander seeds, black peppercorns, mustard seeds, capers... After they're done, sprinkle with grated lemon zest, or parmesan cheese, fried finely diced pancetta, toasted hazelnuts, sesame oil, breadcrumbs sautéed in olive oil, dill, fresh coriander.... the sky's the limit.