To make the dough: In a large bowl, combine the flour, instant dry yeast, sugar, salt, and oil. Add the lukewarm water and mix well. Knead until you have a smooth, pliable dough with an even texture. Slam it against the countertop a couple of times to strengthen the gluten, resulting in a lighter, fluffier dough. Transfer the dough to a lightly greased bowl, cover with cling film, and set aside for 1½ hours. Allowing the dough to rest makes it easier to work with.
Half an hour before baking, place a pizza stone or baking sheet in the oven and preheat it to the highest temperature possible. You want it screaming hot! This ultra-hot environment is what gives you the kind of crust you would expect from a bakery.
Meanwhile, prepare the topping: Finely dice the red bell pepper, green bell pepper, tomato, garlic cloves, red chili pepper (if using), and parsley. Mix everything together, then finely chop the mixture once more with a chef's knife until you achieve a chunky purée.
Now add the meat, tomato paste, sumac, salt, ground cumin, paprika, black pepper, and red chili pepper (if using). Mix well until all the ingredients are thoroughly combined. The mixture should have a paste-like consistency.
Once the dough has rested, lightly dust your work surface with flour. Divide the dough into 8 equal portions and shape each portion into a ball. Cover with a kitchen towel and set aside to rest for 15 minutes.
Dust your rolling pin with flour and roll out one dough ball on a lightly floured countertop into a thin oval measuring 6 × 9 inches (15 × 22 cm) and about 1.5 mm thick. Transfer it to a piece of parchment paper or a silicone baking mat, then spread one-eighth of the topping mixture evenly over the dough. Make sure the entire surface, including the edges, is well coated. Carefully transfer the lahmacun to the preheated baking sheet in the oven, taking care not to burn your hands.
Bake for 8–9 minutes, until the edges are lightly browned and crispy. Repeat with the remaining dough and topping mixture.
To serve: Top your lahmacun with a handful of arugula or parsley and drizzle with freshly squeezed lemon juice for a serious punch of flavor. Add a few onion wedges, wrap it up, and serve piping hot straight from the oven.