Position a rack in the lower third of an oven and preheat to 400°F
Season the chicken thighs on both sides with salt, pepper, cumin and coriander. Heat the oil in a large ovenproof skillet over medium-high heat. Add the chicken, skin side down, and cook until the fat has rendered and the skin is crisp and golden brown, about 8 minutes.
Flip the chicken to the other side and scatter the lemon slices on top. Transfer to the oven and roast until an instant-read thermometer inserted into the thickest part of a thigh, away from the bone, registers 170°F (77°C), 18 to 20 minutes.
While the chicken is roasting, make the smashed peas. Transfer the garlic to a medium bowl and add the pecorino, olive oil, lemon juice, red pepper flakes, salt, and plenty of black pepper, and whisk to combine. Add the thawed peas and stir to combine. Taste and adjust salt and pepper as needed. Use the back of a fork and just roughly mash half of the peas so they are a little more chunky and leave the other half whole.
Remove the chicken from the oven, add the peas into the ovenproof skillet, basically nesting the chicken thighs into the peas and coating the peas with the chicken renderings and top with herbs and extra lemon, salt and pepper.