Preheat the oven to 380ºF.
In a food processor, chop the onion finely. Add 1 cup black beans, cilantro, parsley, mint and cumin, and pulse to combine.
Transfer the mixture to a medium bowl. Add the remaining black beans and egg, season with salt and pepper, then mix with a spoon until well combined. Refrigerate.
Prepare the tahini. (Do not use store-bought, ready-to-eat tahini. It will separate in cooking.) Put the tahini ingredients in a small bowl and gently whisk them. The mixture might be lumpy at first, but don't worry; keep stirring until it becomes smooth and velvety. If it is too thick, add a little more cold water; if it's too runny, add a little bit tahini butter.
Grease a pie dish with oil. Put the black bean mixture in the pie dish and flatten it with oily fingers or with a spatula. Put it in the oven and bake for 15 minutes. Pour the tahini over and put it back in the oven for 5 minutes.
Let it cool for 10 minutes then sprinkle the microgreens over. Serve with a crunchy romaine salad on the side.