Begin by preparing the filling. Heat the oil in a non-stick frying pan, and cook the onion and garlic over a fairly low heat for around 5 minutes, until soft and translucent. Remove from the heat, and allow to cool for a couple of minutes. Then, transfer the mixture to a large bowl and add the watercress, ricotta, yogurt and agave nectar. Mix well and season to taste (remember, the watercress is quite peppery, so go easy on the black pepper!).
Next, prepare your crepe batter. Using a whisk, beat the eggs into the flour, and then add the milk a little at a time, whisking until smooth each time before adding more.
Give the frying pan a quick wipe with some kitchen roll (no need to wash it thoroughly), add the butter, and turn on the heat. Once it has melted, turn the heat down quite low, and pour the excess butter back into the pancake mixture, whisking a little to prevent lumps forming. Now, as long as you are using a good quality non-stick pan, you should be able to cook the whole batch of pancakes without adding any extra butter. If your pan is not non-stick, you may need to add a little extra butter before you cook each pancake.
Add a few tablespoons of the pancake mixture to the pan, and swirl the pan so that the batter covers the base in a thin layer. Cook over a fairly low heat for a few minutes. After a minute or so, the edges of the crepe will begin to lift up - a couple of minutes later, the underside will be golden brown. At this point, flip the pancake (I do this slowly with a spatula!) and cook the other side for another minute or two. Transfer to a plate (in a slightly warm oven, if you want to keep them warm) while you repeat with the remaining crepe batter.
Just before you are ready to serve, add the balsamic to a small saucepan, and bring to a gentle simmer. Cook for a few minutes, stirring regularly, until the mixture has approximately halved in volume (it will continue to thicken as it cools, so it's better to remove it from the heat when it's still just slightly too thin).
Add a few tablespoons of the ricotta mixture to one quarter of each crepe, and fold them in half, then in half again. Serve with a drizzle of the balsamic reduction.