Preheat the oven to 425°F(220°C) and line a large baking pan with parchment paper.
In a large bowl, whisk together the flour, cornmeal, baking powder, onion powder, salt, dill, green onions, and cilantro.
Add the cold, cubed butter and use a pastry cutter to cut the butter into the flour until pea sized lumps form.
Pour in 250 ml (1 cup) of buttermilk and use a silicone spatula to mix and fold everything together until a shaggy dough forms.
Tip the dough out onto a lightly floured surface. With floured hands, quickly squish and shape the dough into a ball. Be careful not to overwork the dough. It will be very crumbly at first but it will come together eventually.
Sprinkle the top of the dough with a bit of flour and use a rolling pin to roll it into a ¾-inch (2 cm) thick horizontal rectangle. Take one of the short ends of the dough and fold it into the center. Take the other and fold it over the entire thing. Repeat this entire step two more times to create layers in the dough.
Roll the dough into a ¾-inch (2 cm) thick rectangle once more and straighten out the sides with your hands. Use a bench scraper or sharp knife to cut the dough into 8 equal sized rectangular biscuits (cut straight down, don't saw).
Place the biscuits on the prepared baking sheet and brush the tops with 2 tbsp of buttermilk. Bake for 28-30 minutes until the tops are golden brown.
Immediately brush the freshly baked biscuits with the melted butter and sprinkle the tops with flaky sea salt.