Turn your oven to 375.
If you're using kale, steam it to soften. If your beans are fresh not frozen, optional to steam them too until softened (8-10 minutes).
Make a roux-- in a small sauce pan, melt your butter, and then whisk in the flour to make a paste. Cook for a minute or two until the smell of flour has disappeared. In a separate pot, warm up your milk/stock/cream-- it just needs to be warmed, not boiling, so keep an eye on it. When it's warmed up, whisk it into your flour/butter mixture over low heat. Keep whisking to thicken. Add in a few dashes of Worcestershire sauce, salt and pepper.
Turn off the heat on the roux, and mix in the eggs and the red onions, nutmeg, and the cayenne or Harissa.
Gently begin to mix in the cheese (save about 1/2 cup for the top), the greens and the beans all together into the pot with the roux.
Transfer everything to a greased baking dish, and top with the reserved cheese. Bake until a golden crust has developed, about 30-40 minutes.