Heat two tablespoons of oil in a sauté pan over medium heat. Add the mushrooms and cook them until they're tender and shiny brown, stirring occasionally. Sprinkle them with salt.
Remove the mushrooms and pour another two tablespoons of olive oil into the pan. Add the onions and cook them until they're tender and golden, stirring occasionally. Sprinkle them with salt. You may want to lower the heat so they don’t burn too quickly before they become golden.
At this point you can combine the mushrooms and onions in a container to refrigerate them overnight, or put them on the side to cool and move forward with your biscuits. If you’re going to make the biscuits right away, preheat your oven to 425 degrees. If you’re in a hot kitchen consider putting the veggies in the fridge, because hot mushrooms and onions will melt the butter in the biscuit dough.
To make the biscuits, mix flour, sugar, baking powder, baking soda and salt in a bowl. Cut in the butter (with a knife, fork or your fingers) until the mixture resembles a coarse meal.
Crack the egg into the measuring cup with your buttermilk, and beat the two lightly together. Add the mushroom and onion mixture to the dough and combine them together.
Pour the mixture into the bowl with the dry ingredients and stir to make a soft dough.
Turn the dough onto a floured countertop and knead gently. At this point, I actually had to put mine back into the fridge because the butter was melting in my 90 degree kitchen, so if that’s what you need to do then don’t be afraid! Anyway, once your dough is firm enough to handle, knead it about 15 times.
Roll or pat it out to about 1-inch thickness, and cut it into two-inch rounds using a biscuit cutter. Alternatively you could do what I did and cut around the lid of a mason jar. You could also cut them into squares.
Place the rounds onto an ungreased baking sheet and brush them with the egg wash. Joy suggests sprinkling them with coarse sea salt, which I wanted to do but I didn’t have any. I’m sure it would make the flavour really pop.
Bake them for 12 - 15 minutes. Serve warm.