Step 1: Marinate the Chicken

1. Prepare the Marinade: In a large bowl, whisk together olive oil, garlic, cumin, smoked paprika, coriander, turmeric, cinnamon, cayenne (if using), salt, black pepper, and lemon juice.
2. Coat the Chicken: Add the chicken thighs to the bowl, tossing to coat evenly. Cover and marinate for at least 15 minutes, or up to 24 hours in the refrigerator for deeper flavor.

Step 2: Prepare the Vegetables

1. Season the Vegetables: In a separate bowl, toss the sliced onions and bell peppers with olive oil, salt, and black pepper until evenly coated.

Step 3: Roast Everything on a Sheet Pan

1. Preheat the Oven: Preheat your oven to 425°F (220°C) and line a large sheet pan with parchment paper or aluminum foil for easy cleanup.
2. Arrange the Ingredients: Spread the marinated chicken thighs on one side of the sheet pan and the seasoned vegetables on the other. Ensure everything is in a single layer for even roasting.
3. Roast: Bake for 25-30 minutes, flipping the chicken halfway through, until the chicken reaches an internal temperature of 165°F (74°C) and the vegetables are tender and slightly caramelized.

Step 4: Slice the Chicken

1. Rest and Slice: Let the roasted chicken rest for 5 minutes, then slice into thin strips.

Step 5: Assemble the Sandwiches

1. Toast the Bread: Lightly toast the sandwich rolls or buns under the broiler for 1-2 minutes until golden.
2. Layer the Ingredients: Spread a generous layer of tzatziki sauce on the bottom half of each roll. Add shredded lettuce, sliced tomato, roasted vegetables, sliced chicken, and two slices of Havarti cheese.
3. Melt the Cheese: Place the assembled sandwiches under the broiler for 1-2 minutes, just until the Havarti cheese melts and becomes gooey.
4. Top and Serve: Add the top half of the roll, slice the sandwich in half if desired, and serve warm.