Shawarma Roast Chicken With Shallots and Lemons

Name: Shawarma Roast Chicken With Shallots and Lemons
Ingredients: 1 Tbsp. coriander seeds; 1½ tsp. cumin seeds; 1½ tsp. black peppercorns; 1½ tsp. smoked paprika; ¾ tsp. ground turmeric; 4 garlic cloves, finely grated; ¼ cup whole-milk plain Greek yogurt or sour cream; 4 Tbsp. extra-virgin olive oil, divided; 1 3½–4-lb. whole chicken, patted dry; Kosher salt; 3 medium shallots, halved lengthwise; 1 lemon, sliced into ¼-thick rounds, seeds removed; 4 oregano or thyme sprigs, plus leaves for serving; Flaky sea salt (for serving); A spice mill