Heat olive oil in a very large frypan or paella pan over medium-high heat until it begins to smoke. Place chicken thighs skin-side down into oil and sear until golden brown on both sides; about 5 minutes. Set aside.
Stir in onion and garlic then cook until the onion has softened and turned translucent; about 1 minute. Add green and red capsicums, calamari and small prawns; cook for 2 minutes more.
Stir in salt, saffron, tomatoes, peas, rice and water until well combined. Add chicken thighs then simmer for 15 minutes over medium-high heat stirring frequently to make sure rice does not stick.
Decoratively nestle clams and jumbo prawns into the rice. Reduce heat to medium-low then cover and simmer for 10 minutes.
Place the scallops into the paella then recover and continue simmering until the rice is tender and the scallops firm and opaque; about 5 minutes.