1 cup coarsely chopped chives, plus 2 tablespoons finely chopped
1/3 cup grapeseed or canola oil
1 1/2 pounds Yukon Gold potatoes, peeled and cut into 2-inch chunks
5 tablespoons unsalted butter
2/3 cup half-and-half
2/3 cup shelled fresh or frozen soybeans (edamame)
1/2 cup coarse, dry rye-bread crumbs
Four 6-ounce black sea bass fillets
Blend the coarsely chopped chives with the oil and a generous pinch of salt at high speed until smooth. Transfer the chive oil to a jar.
In a large saucepan, cover the potatoes with cold water and bring to a boil. Salt the water and simmer the potatoes until tender, about 20 minutes. Drain thoroughly. Pass the potatoes through a ricer or the fine disk of a food mill back into the saucepan.
In a small saucepan, melt 2 tablespoons of the butter in the half-and-half. Add to the potatoes, season with salt and stir until smooth. Stir in all but 2 tablespoons of the chive oil and keep warm over very low heat.
Bring a small saucepan of salted water to a boil. Add the soybeans and boil over high heat for 5 minutes. Drain and let cool, then finely chop.
Melt 2 tablespoons of the butter in a small skillet. Add the chopped soybeans and cook over moderately high heat until sizzling, about 2 minutes. Stir in the bread crumbs and the 2 tablespoons of finely chopped chives and season with salt.
Season the fish on both sides with salt and cayenne. In a large nonstick skillet, melt the remaining 1 tablespoon of butter over moderately high heat. Add the fish, skin side up, and cook until golden brown, about 5 minutes. Turn the fish and cook for 1 minute longer, or until just cooked through. Turn the fish again and spoon the soybean crust on top of each fillet, pressing lightly to adhere.
Mound the mashed potatoes in the center of each plate and set the fish on top. Drizzle the remaining chive oil around the fish and serve.