How to Make It
Preheat the oven to 450°. Set the dried-out rock in the center of a large enameled cast-iron casserole and bake for 1 hour, until the rock is very hot.
Meanwhile, in a large saucepan, cover the kombu with 8 cups of hot water and let soak, turning occasionally, until rehydrated and pliable, about 30 minutes. Lift out the kombu; reserve 2 cups of the kombu soaking water.
Fill a large bowl with ice and water. In a large pot of boiling water, blanch the lobsters head-first for 1 minute. Using tongs, transfer the lobsters to the ice-water bath and let soak until chilled, about 10 minutes. Drain well. Twist off the arms and claws. Twist the lobster bodies to detach the heads from the tails. Discard the lobster heads (or save for bisque).
Spread the wood chips in a heavy cast-iron skillet and toast over high heat, without stirring, until they begin to smoke, 3 minutes. Transfer the chips to a small heatproof bowl to cool.
Wipe out the skillet. Add the lemons cut side down and cook over moderately high heat until almost blackened, about 5 minutes. Transfer to a plate.
Sprinkle the wood chips around the hot rock in the casserole. Line the casserole with the kombu, overlapping it slightly, to cover the rock; allow the ends to hang over the edge. Arrange the lobster tails, arms and claws around the casserole along with the clams, mussels and chorizo. Add the spring onions and mushroom clusters and sprinkle with 2 tablespoons of the chopped garlic chives. Pour in the reserved 2 cups of kombu soaking water and fold the overhanging kombu over everything in the casserole.
Cover with the lid. Cook the clambake in the center of the oven for 20 minutes; the lobster should be bright red and the clams and mussels should be open. Remove the clambake from the oven and let stand, covered, for 5 minutes.
In a blender, combine the chorizo and olive oil and blend until the chorizo is minced; scrape into a small saucepan and bring to a simmer over low heat. Remove from the heat and let stand for 20 minutes, until the oil is bright red and infused with chorizo flavor, about 20 minutes. Strain the oil into a bowl and stir in the minced garlic chives, then fold in the salmon roe.
Sprinkle the remaining 2 tablespoons of chopped garlic chives over the clambake and serve with the charred lemon halves, Salmon Roe, Garlic Chive and Chorizo Oil, steamed new potatoes and corn on the cob.
To make absolutely sure that the rock is dry all the way through, set it on a baking sheet and bake at 200° for 10 hours.