Mash garlic and ½ teaspoon salt on a cutting board with the back of a fork until a paste forms. Transfer to a medium bowl and whisk in lemon juice, cumin, paprika and pepper. Add chicken and stir to coat.
Heat oil in a large cast-iron skillet over medium-high heat. Add onion and cook, stirring occasionally, until golden brown, 6 to 8 minutes. Using a slotted spoon, transfer the chicken to the pan (reserve the marinade) and cook, stirring occasionally, until opaque on the outside, about 4 minutes. Add tomatoes with their juice, chickpeas, the reserved marinade and the remaining ¼ teaspoon salt. Reduce heat to medium and cook, stirring occasionally, until the chicken is cooked through, 5 to 7 minutes more. Serve sprinkled with parsley.
