1. Rinse the beans and sort to remove any foreign objects.
2. Put in a large pot, cover with plenty of water, and let stand overnight.
3. The next day, add a pinch of baking soda to the water (if you live in a hard water area), and simmer the beans until just tender. The cooking time may be as little as 30 minutes, or over an hour, depending on the beans. Add additional water if necessary.
Once cooked, drain well and toss them in a bit of olive oil, which will prevent the skin of the beans from flaking, and bit of salt.
4. Next up is frying the beans: You’ll need to cook the beans in a single layer, so doing a rough calculation on depending on the size of your skillet, heat a portion of the butter and oil in the pan. Once hot, add enough beans so they’re spread out evenly in a single layer in the pan.
Saute the beans and avoid stirring too much so they brown and blister on the bottom side. After a few minutes, stir them a bit to flip them around, and continue to cook a few more minutes until they’re all nicely seared.
5. Once finished, transfer the beans to a bowl and fry the remaining beans in a similar fashion.
When you get to the last batch, during the final minute of cooking, add the spring onions, garlic, and sorrel. Cook until the greens are just wilted, which will take another minute or so. Remove from heat, add the other beans to the pan, stir, then add the 1/2 teaspoon of salt. Let cool to room temperature, or until slightly warm.
(If your pan isn’t large enough, you can mix everything together in a large bowl.)
6. Once cooled, stir the lemon juice and sumac, and then roughly chop the fresh herbs and mix them into the beans. Crumble the feta over the top and serve.