Heat olive oil in a paella pan over medium-high heat. Season the chicken thighs with salt and black pepper, then cook skin side down until deeply golden and crispy, about 8 minutes.
Flip the chicken and cook the second side for 3 minutes until lightly browned. Transfer to a plate while keeping the juices.
Add onion, red bell pepper, and green beans to the pan. Sauté for about 5 minutes until softened and lightly caramelized.
Stir in the sliced garlic and cook for 1 minute until fragrant. Add crushed tomatoes and smoked paprika, then cook until the tomato mixture thickens slightly and darkens in color.
Pour in the chicken stock and add saffron threads and bay leaf. Bring the broth to a gentle simmer.
Stir in the bomba rice and spread it evenly across the pan using a wooden spoon. Return the chicken thighs to the pan with any accumulated juices.
Reduce the heat to medium-low and simmer without stirring for 10 minutes. Add the frozen peas and rosemary sprig, then continue cooking for another 10 to 15 minutes until the rice is tender.
Listen for a sharper sizzling sound near the end of cooking to check for socarrat forming underneath. If needed, increase the heat briefly for 1 to 2 minutes to create the crispy crust.
Remove the pan from the heat and let the paella rest uncovered for 5 minutes. Garnish with parsley and serve with lemon wedges.
