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1
Fillet the tench, reserving all bones, the head and the fins. Place the carcass, head and fins in a pot with the halved onion, chopped celery, bay leaves, fennel fronds and white wine, then cover with cold water, starting the pot cold so the broth rises to clarity rather than turning cloudy from a sudden boil. Bring to a bare simmer and cook for 30 minutes, then strain through a fine sieve, discarding all solids, and keep the broth at a bare simmer; it is ready when it is clear, pale gold and carries a clean, mineral lake-fish scent.
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2
Lay the tench fillets in a shallow dish and pour over the white wine and lemon juice, so the acid draws out the raw earthy lake-bottom note that tench carries from freshwater — leave them for 1 hour. The marinade is complete when the flesh has firmed very slightly at the edges and that muddy scent has retreated.
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3
Warm the olive oil in a wide, heavy saucepan over medium heat, add the finely chopped onion and sweat gently until completely soft and translucent with no colour, so the onion sweetens into the base without any caramel bitterness competing with the fish. Add the rice and toast, stirring constantly, until each grain is coated and the pan gives off a dry, faintly nutty scent — about 2 minutes.
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4
Pour in the white wine and stir until it has been entirely absorbed, so the sharpness of raw spirit cooks off and leaves only the wine's clean fruit behind. The deglazing is done when the sizzle quietens and the pan smells of rice and wine rather than raw spirit.
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5
Add the simmering tench broth one ladle at a time, stirring continuously and allowing each addition to be absorbed before adding the next, so the rice releases its starch gradually and the risotto builds a body that carries the lake-fish flavour through every grain — too fast and the rice drowns; too slow and it seizes. Continue for approximately 16–18 minutes; the risotto is ready when the grains are tender but retain a ghost of resistance at the centre and the mass flows slowly when the pan is tilted.
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6
Remove the pan from the heat and add the cold cubed butter, beating it vigorously into the risotto so the sudden chill and agitation bind the butter into the starchy mass, giving a gloss and body that warm butter cannot achieve. No Parmigiano is added, as cheese overwhelms the delicate freshwater fish; the mantecatura is complete when the surface shines and a spoonful dropped back into the pan settles slowly rather than immediately.
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7
While the risotto rests for 1 minute, heat a wide frying pan over medium-high heat and melt the 80 g of butter until it foams, subsides and turns a deep hazelnut colour, then add the sage leaves so they crackle and crisp in the hot fat and release their oils directly into the butter. Pat the marinated tench fillets dry before laying them skin-side down in the brown butter — drying the surface ensures the skin colours rather than steams — and cook for 2 minutes per side; the fillets are done when the flesh is just opaque through to the centre and lifts cleanly from the pan, since overcooked tench turns dry and its earthiness dominates.
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8
Spoon the risotto into four warmed shallow bowls and lay the tench fillets over the risotto so they remain intact. Pour the brown butter and sage from the pan over both fish and risotto, then grate lemon zest directly over each portion at the moment of service so the volatile oils lift immediately on the heat of the dish rather than fading in preparation. The plate is ready when the butter glistens on the fish and the scent of brown butter, sage and lemon rises together.