For the pancakes, sift the flour and salt into a large bowl and make a well in the centre. Break the eggs into the well, then gradually mix the eggs into the flour with a fork. Gradually add the milk and water to the mixture, whisking continuously, until the mixture is smooth and has the texture of double cream. Finally, whisk in the butter.
Heat a drizzle of the oil in a 18cm/7in non-stick frying pan over a high heat until smoking, then turn the heat down to medium. Spoon in about two tablespoons of the pancake batter into the pan, swirling it around to evenly coat the bottom of the pan.
Cook for about 30 seconds, then flip the pancake over and cook for a further 15-20 seconds, or until cooked through. Place the pancake onto a plate lined with greaseproof paper, then place another layer of greaseproof paper on top. Repeat the cooking process with the remaining pancake batter, adding more oil to the pan if necessary, and making sure each pancake sits between two layers of greaseproof paper to prevent them from sticking to each other. Set aside.
Preheat the oven to 180C/350F/Gas 4.
For the filling, melt the butter in a pan until foaming, then gently fry the leeks for 4-5 minutes, or until softened. Drain well, then place into a bowl.
Add the smoked cheese, thyme, sage, peas and ricotta to the bowl of leeks and mix together until well combined.
Spread about three tablespoons of the filling over one pancake. Roll up the bottom, then the two sides; roll the pancake upwards to form a cylindrical shape like a spring roll. Repeat with the remaining filling and pancakes.
Dredge each pancake roll in flour, then dip into the beaten egg and roll in the polenta to coat.
Heat a drizzle of oil in a frying pan and fry the pancake rolls for 3-4 minutes on each side, or until golden-brown all over. Place the pancakes into the oven to cook for 10-12 minutes, or until heated through. Set aside.
For the salsa, mix together all of the salsa ingredients in a bowl.
To serve, arrange the pancake rolls on serving plates with the salsa in small bowls alongside for dipping.