To make the base, mix the flour, yeast, salt and sugar in a large bowl. Add the oil and just enough warm water to form a soft dough. Tip on to a lightly floured surface and knead for 10 minutes. Place the dough in a lightly oiled bowl, cover and leave to stand until doubled in size – about 1 hour.
Place the pepper, tomatoes, garlic cloves and red onion in a blender and blend until everything is finely chopped. Place in a bowl with the sumac, cumin, aleppo, salt and pepper and minced beef. Mix together with your hands until everything is combined.
Preheat the oven to 200°C. Divide the dough into 8 equal pieces. Roll each portion out on a lightly floured surface to a thin, roughly 15cm-diameter disc. Spread the beef mixture evenly over the top of each one leaving a border around the edge. Place the lahmacun onto greased baking trays and place in the oven for 5-6 minutes until crispy and slightly browned around the edges. You may need to do this in batches.