Marinate Chicken: Combine chicken thigh fillets with salt, sweet paprika, garlic powder, onion powder, 2 tbsp olive oil, 1 tbsp lemon juice, and black pepper. Mix well, cover with cling wrap, and refrigerate for about 30 minutes.
Cook Chicken: Heat 2-3 tbsp olive oil in a pan over medium-high heat until hot. Add chicken and cook for 5 minutes on one side until sizzling and golden. Flip chicken, cover with a lid, and cook for another 5 minutes.
Add Rosemary: After 3 minutes of cooking the second side, remove lid, brush chicken with fresh rosemary, replace lid, and cook for an additional 2 minutes until chicken is cooked through (internal temperature 74°C). Remove chicken and let rest.
Prepare Lemon Butter Sauce with Broccolini: Without washing the pan, add 2 tbsp salted butter to the remaining chicken juices over medium heat. Add broccolini and cook together, allowing broccolini to absorb the flavors. Turn off heat, add 1 tbsp lemon juice and cracked pepper, stirring to create a caramelized lemon butter sauce.
Serve: Plate the broccolini as a bed in the center of the plate, place chicken thighs on top, garnish with a sprig of rosemary, and drizzle with the lemon butter sauce.