Start by making the pesto. Put all the ingredients apart from the olive oil in a blender and blend until chopped and combined. With the motor running stream in the olive oil until fully mixed. Set aside until ready to use. This recipe will make more than you need so you can store leftovers in the fridge for up to 5 days.
Put the butter and olive oil in a frying pan over a medium high heat. Once the butter is melted add the mushrooms and cook until golden brown and crisp. Use a slotted spoon to scoop the mushrooms onto a paper towel lined plate.
Put the cavolo nero into the same pan and add a pinch of salt, the chilli flakes and 50ml of water. Cover the with a lid and let steam for a couple of minutes until wilted.
Toast the bread on a griddle or in a toaster while the mushrooms and cavolo nero cook. Spread each slice of toast with some of the pesto, top with mushrooms, cavolo nero and sprinkle over the feta.
Nutrition information is an estimate and will vary depending on the exact ingredients used.