Bring bone broth or chicken stock to a simmer in a small saucepan and keep warm over low heat.
Heat oil in a large saucepan or dutch oven over medium heat. Add leek and shallots to pan, cook until tender, about 7 minutes. Add rice and cook 1 minute, until it becomes translucent. Add vermouth and allow to evaporate, about 30 seconds. Stir in 1 ladle of stock and cook until the liquid is nearly absorbed, stirring constantly. Add remaining stock, 1 ladle at a time, stirring constantly until each portion of stock is absorbed before adding the next (about 25 minutes total). Stir in frozen peas and cook 1 minute. Remove from heat. Stir in parmesan, lemon zest, lemon juice, tarragon and butter. Season to taste with salt and pepper.
In a large skillet, heat 1/2 cup of oil until shimmering. Pat the scallops dry and season with salt and pepper. Add the scallops to the skillet and cook over high heat until browned on the bottom, about 3 minutes. Add the butter to the skillet. Turn the scallops over and cook just until white throughout, 2 minutes longer.
To serve: Plate the risotto into four bowls, top each with three of the seared scallops. Serve immediately.