Place the chicken thighs between two sheets of plastic wrap or parchment paper and lightly pound the thicker parts to an even thickness.
In a large bowl, whisk together the lemon juice, olive oil, pepper paste, garlic, salt, black pepper, paprika, cumin, thyme, oregano, and cayenne pepper.
Add the chicken thighs and toss until evenly coated. Cover and marinate for at least 30 minutes. For best flavor, refrigerate for 1 hour or up to overnight.
Preheat the oven to 425°F (220°C).
Place one onion half, cut side down, on a baking sheet. Insert two skewers about 2 inches apart through the center of the onion to create a stable base. Stack the marinated chicken thighs onto the skewers. If desired, top the stack with the remaining onion half or a tomato.
Pour 1 cup water into the baking sheet around the onion. As the shawarma bakes, the water will combine with the marinade and chicken juices to create a flavorful pan sauce. Check occasionally and add more water if needed to prevent the pan from drying out.
Bake for 60–90 minutes, or until the chicken reaches an internal temperature of 165°F (74°C). Cooking time may vary depending on the size of the chicken thighs and how tightly they are stacked. Occasionally spoon the pan sauce over the chicken to keep it moist and flavorful.
Slice the chicken thinly from the outside of the stack and spread it onto a baking sheet. Drizzle some of the pan sauce over the sliced chicken, then broil for 3–5 minutes, or until the edges are lightly charred and crispy, watching carefully to prevent burning.
Serve in pita or flatbread with tomatoes, cucumbers, sumac onion, pickles, and garlic sauce. Bon appétit.