Heat the butter in a large pan over medium-high heat.
Add the sliced leeks and fry for 5 to 6 minutes until soft but not browning.
Remove the leeks from the heat and set them aside to cool.
To make the accompanying garlic and chive yogurt, combine the yogurt, minced garlic, lemon juice, 1 tablespoon of olive oil, the chopped chives, and salt and pepper, to taste. Set aside.
In a large bowl, whisk together the eggs, mustard powder, flour, baking powder, and milk to form a batter.
Stir in the cooled leeks, the chopped parsley, chopped tarragon, and the grated cheddar, and season to taste to complete your fritter batter.
Heat the remaining 2 tablespoons of olive oil in a large pan over medium heat and dollop tablespoon-sized amounts of the fritter batter into the hot pan.
Allow the batter to cook for 3 to 4 minutes until the bottoms are crisp, and then flip the fritters over and cook them for another 3 to 4 minutes on the other side.
Remove cooked fritters from the pan and repeat this process until all of the batter has been used up.
Serve the fritters warm with a spoonful or two of the garlic and chive yogurt.