To make the purée, melt 1 1/2 tablespoons butter in a saucepan, add the celeriac, and cook over a medium-high heat until starting to brown a little, about 5 mins. Pour over the milk and cream, and season well—the celeriac should be just covered, so add more milk if needed. Bring to a simmer, cover and cook for 15 mins or until tender. Drain, reserving the cooking liquid, and transfer to a food processor. Blend until smooth, then sieve into a clean saucepan, pushing the celeriac through with the back of a spoon or spatula. Add a little of the cooking liquid to loosen the purée if needed, and taste for seasoning.

To make the crumbs, heat 1 1/2 tablespoons of butter in a frying pan, add the breadcrumbs and cook, stirring, until golden brown. Throw in the capers and cook for 30 seconds more. Tip into a bowl and leave to cool a little, then stir in the parsley.

Heat oven to 300 degrees F. Gently melt the butter in a small ovenproof saucepan or frying pan, big enough for the fish fillets to fit snugly. Once melted, add the shallot, bay leaf and peppercorns. Keep heating until the butter turns a light brown color and smells nutty. Leave to cool a little. Gently lower the barramundi fillets into the warm butter, basting any exposed fish, and transfer to the oven. Cook for 3-4 mins, depending on the thickness of the fish, then turn the fillets over and cook for a further 3-4 mins or until just opaque and feeling firm.

While the fish cooks, reheat the purée and steam the greens. To serve, spoon the purée into the center of each plate; use the back of the spoon to spread out slightly into a circle. Top with the greens, then carefully place the fish on top. Drizzle with a little of the poaching butter and scatter with the crumbs.