I cook either cavolo nero (aka Tuscan kale) or collard greens every week, and I always braise them with onion, olive oil, salt, and water. They’re so good I don’t think I need to try anything else. The secret ingredient is time; the greens are best if cooked for at least 45 minutes.
Chop an onion and start cooking it in enough extra virgin olive oil to cover the bottom of your pan (anything with a decent lid will be fine). While the onion cooks, chiffonade a bunch of greens: roll half the bunch into a tight bundle and cut into quarter inch slices. It isn’t necessary to cut out the central stalk since you’re going to cook them until they’re tender.
Add the greens to the onion along with some salt and about a half cup of water. Cover, reduce heat to simmer, and cook. Check after 20 minutes and add water to keep the bottom of the pot covered (I’ve burned greens more than once; sometimes you can save them and just say they’re “caramelized). Let them simmer for at least 45 minutes, longer is okay (but check for water). Drizzle with a bit of fresh extra virgin at the table. Really good with beans.