Heat your oven to 190°C/Fan 170°C/Gas 5. Tip the flour into a large bowl (save 1 tbsp for later). Add a pinch of salt. Crack the egg and tip the white into 1 bowl and the yolk into 1 bowl. Whisk the yolk with 2 tbsp oil. Add to the flour with 50ml cold water. Stir with a fork to make a soft dough. Add more water if needed (make sure it isn't too wet). Turn out and knead to a smooth pastry. Cover with cling film and put in the fridge.
Peel and finely chop the onion. Heat a large frying pan. Add ½ tbsp oil and the onion. Season. Fry gently for 3 mins. Add the mince and fry for 5 mins. Stir to break up any lumps.
While the mince cooks, peel and crush the garlic. Finely chop the tomatoes. Halve the chilli and flick out some of the seeds. Finely chop it. Add the garlic, tomatoes and chillies to the beef. Stir together and cook for 2-3 mins. Take off the heat.
Scrub the sweet potato and coarsely grate it. Put the pan back on the heat. Add the sweet potato, 1 tsp each smoked paprika and sugar and 1 tbsp vinegar. Simmer for 5 mins till the sweet potato has softened a little. Taste and season. Drain off any excess liquid. Spoon onto a plate to cool.
Sprinkle your work surface with the reserved flour. Divide the pastry into 4. Roll into saucer-sized circles. Heap the mince onto one half of each circle, leaving a 1cm border. Top each with oregano leaves. Whisk the egg white and brush around the pastry edges. Fold the empty half of the pastry over the filling. Press the edges together and crimp with your fingers . Repeat with the other empanadas. Place on a baking tray lined with baking paper. Brush with the rest of the egg white. Sprinkle ½ tsp sugar over each one. Season. Bake for 20 mins till golden.
Halve the pepper, scoop out the seeds and finely slice. Fry in a frying pan for 5 mins till slightly blackened. Tip into a bowl with 1 tsp paprika and the remaining vinegar. Blend till smooth with a hand-held blender, or in a food processor. No processor? Finely chop the peppers and stir with the paprika and vinegar. Serve the empandas with the pepper sauce and salad leaves.