Cut chicken thighs into 1-inch pieces. Pat shrimp and mussels dry. Finely chop the onion, mince garlic, dice the red bell pepper, and halve the green beans. Measure out all spices, rice, and broth. Having everything ready (mise en place) ensures a smooth cooking process, as paella requires quick, sequential additions.

Heat 2 tablespoons of olive oil in your paella pan over medium-high heat. Add the chicken pieces in a single layer and sear for 5-7 minutes until golden brown on all sides. Remove chicken and set aside. Add the remaining 2 tablespoons of olive oil, then add onion, bell pepper, and green beans. Sauté for 5-7 minutes until vegetables soften. Add minced garlic and cook for 1 minute until fragrant.

Stir in the diced tomatoes and cook for 3-5 minutes, allowing their liquid to reduce slightly and flavors to meld. Sprinkle in the sweet smoked paprika and stir for 30 seconds. This 'blooms' the paprika, enhancing its flavor. Add the paella rice to the pan and stir constantly for 2-3 minutes, toasting the grains. This helps the rice absorb flavor and cook evenly.

Warm about 1/4 cup of the broth in a small bowl, then add saffron threads and let them steep for a few minutes to release their color and aroma. Pour the saffron-infused broth and the remaining hot broth into the paella pan. Stir gently to distribute the rice and ingredients evenly. Season with salt and pepper. Bring the liquid to a gentle boil.

Once boiling, reduce heat to medium-low, maintaining a steady simmer. Arrange the reserved seared chicken pieces evenly over the rice. Cook uncovered for 10 minutes without stirring. This allows the rice to absorb liquid and begin cooking, forming the foundation of the paella.

Nestle the shrimp and mussels into the partially cooked rice, ensuring mussels are hinge-side down. Continue to simmer for another 10-15 minutes, or until the rice is tender, most of the liquid is absorbed, and the mussels have opened. If needed, you can briefly increase heat for the last 2-3 minutes to encourage the 'socarrat'.

Once the paella is cooked, remove it from the heat. Cover the pan loosely with foil or a clean kitchen towel and let it rest for 5-10 minutes. This allows the flavors to meld and the rice to finish cooking evenly. Garnish with fresh chopped parsley and lemon wedges before serving directly from the pan.