Pat the chicken thighs dry and season both sides with kosher or sea salt and 1/2 teaspoon of the black pepper.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add half of the chicken and brown about 3 minutes per side until golden; transfer to the slow cooker.
Add the remaining 1 tablespoon olive oil to the skillet and brown the remaining chicken, about 3 minutes per side; transfer to the slow cooker.
In a bowl, whisk together the chicken broth, lemon juice, flour, and ground cumin until smooth.
Pour the broth mixture over the browned chicken in the slow cooker. Scatter the pitted green olives on top and sprinkle the remaining 1/4 teaspoon black pepper over everything.
Cover and cook on low for 6 hours, until chicken is tender and cooked through.
If using, add lemon slices as a garnish before serving.
