Find a 12 hole muffin tin
Heat oven to 220º C
Sauté onion until softened
Add mushroom and cook until softened
Add stock cube and cook until melted
Beat the eggs in a large bowl
Add the mustard, mushroom mix, grated cheddar cheese, salt and pepper to the eggs mixture
Cover the bowl with cling film and refrigerate until needed.
Cut the sheet of puff pastry into 12 even pieces
Grease the muffin tin
Press a piece of pastry into each muffin tin hole ensuring you cover the bottom and sides of the hole
Spoon the mushroom and cheese mixture into the the holes (fill to the top)
Bake until golden brown on top (~20-25 minutes)
Remove from the oven
After 5 minutes carefully take the tarts from the tins and place on a cooling rack