Sheet pan kebabs

For the meat: 
2 lb 3 oz (1 kg) ground beef or lamb (15-20% fat)
1 small brown onion, grated
1 tbsp fresh garlic, minced
1 tbsp ground cumin
1 tbsp ground coriander
1 tbsp sweet paprika
1 tsp chili flakes (optional)
2 tsp sea salt flakes
¼ tsp freshly ground black pepper
3 tbsp flat-leaf parsley, finely chopped
½ tsp baking soda
2 tbsp olive oil, for basting

For the sumac onions: 
1 small red onion, thinly sliced
1½ tsp sumac
Juice of ½ lemon
1 tsp olive oil
¼ tsp sea salt flakes

For the garlic yogurt sauce: 
1¼ cups (310 g) Greek yogurt
½ tsp fresh garlic, minced
1½ tbsp lemon juice
½ tsp sea salt flakes

To serve: 
4 to 6 flatbreads or pitas
2 large tomatoes, cut into wedges or thinly sliced rounds
½ cup (15 g) fresh parsley, finely chopped

Preheat the ovento 240 °C (475 °F), or 220 °C (425 °F) for convection ovens.

Prepare the meat mixture: In a large bowl, combine the ground beef, grated onion (coarse grater), garlic, cumin, ground coriander, paprika, chili flakes (if using), salt, pepper, parsley, and baking soda. Mix by hand for about 2 minutes, until the mixture becomes sticky and cohesive.

Shape on the baking sheet: Line a large baking sheet with parchment paper. Press the meat mixture onto the sheet to form a large rectangle about 1 cm (½ in.) thick, roughly 30 x 40 cm (12 x 16 in.).

Score and indent: Using a bench scraper or metal spatula, slice the mixture vertically into strips 2 to 3 cm (¾ to 1¼ in.) wide, cutting all the way to the paper. Then make shallow horizontal indentations on each strip, to mimic the appearance of a traditional kebab and promote even browning.

Bake: Brush the top of the mixture with olive oil. Bake on the top rack of the oven for 15 to 18 minutes, or until the meat is golden brown and fully cooked, with crispy edges. For more pronounced browning, broil for 1 to 2 minutes at the end.

Prepare the sumac onions: While baking, combine the sliced red onion, sumac, lemon juice, olive oil, and salt in a bowl. Let marinate for 5 to 10 minutes.

Prepare the garlic yogurt sauce: In another bowl, mix the Greek yogurt, garlic, lemon juice, and salt. Taste and adjust seasoning.

Warm the breads: Heat the flatbreads or pitas in a dry skillet (or directly over a gas flame) for 20 to 30 seconds per side.

Assemble and serve: Press the warm breads onto the cooked meat to absorb the cooking juices. Separate the kebab strips along the cut lines. Serve in or on the breads with a generous spoonful of yogurt sauce, a pile of sumac onions, tomato slices, and fresh parsley.