TRIM THE GREENS FROM THE TURNIPS. SCRUB OFF ANY GRIT UNDER RUNNING WATER. (NOTE: IF THE GREENS ARE IN GOOD SHAPE, WASH AND DRY THEM. YOU CAN SAUTÉ THEM WITH THE LEEKS.)
MELT 2 TBSPS BUTTER IN A SMALL SAUCEPAN WITH ABOUT 3 CUPS OF WATER. BRING TO A SIMMER, SALT GENEROUSLY. ADD THE TURNIPS, REDUCE HEAT TO A SIMMER AND POACH GENTLY FOR 5-10 MINUTES, DEPENDING ON THEIR SIZE, TILL THEY'RE EASILY PIERCED WITH A SHARP KNIFE. DRAIN IN A SIEVE UNDER COLD RUNNING WATER AND SET ASIDE.
TRIM OFF THE ROOT ENDS AND DARK GREEN LEAVES FROM THE LEEKS. SLICE LENGTHWISE INTO 1/2-INCH STRIPS. RINSE OFF ANY GRIT UNDER RUNNING WATER.
MELT 2 TBSPS BUTTER WITH 1/2 CUP OF WATER IN A SAUTÉ PAN LARGE ENOUGH TO COOK THE FISH. SPRINKLE WITH A LARGE PINCH OF SALT. SIMMER ON MEDIUM LOW HEAT TILL THE LEEKS ARE TENDER BUT STILL GREEN. REMOVE TO A BOWL. POUR THE WATER FROM THE PAN.
RAISE THE HEAT TO MEDIUM HIGH. DRY THE BASS THOROUGHLY WITH PAPER TOWELS. SALT AND PEPPER. ADD ABOUT 2 TBSPS OF CANOLA OR GRAPE SEED OIL TO THE PAN. WHEN IT'S QUITE HOT, GENTLY LAY IN THE FILLETS, SKIN SIDE DOWN. SHAKE THE PAN SO THEY SLIDE AROUND FOR A MINUTE UNTIL THEY NO LONGER STICK TO THE PAN. REDUCE HEAT TO MEDIUM. COOK FOR ABOUT 4 MINUTES, SHAKING THE PAN FROM TIME TO TIME.
TOSS IN SEVERAL SPRIGS OF THYME AND 3 TBSPS BUTTER. BASTE THE FISH. COVER FOR A MINUTE. UNCOVER AND CONTINUE BASTING TILL A KNIFE INSERTED IN THE CENTER IS WARM WHEN TOUCHED TO YOUR BOTTOM LIP. REMOVE THE FISH TO A WARM PLATE.
ADD THE LEEKS TO THE PAN FOR 30 SECONDS TO RE-HEAT THEM. REMOVE AND TOSS WITH THE JULIENNED RADISHES. DIVIDE ONTO TWO DINNER PLATES. PLACE FILLETS, SKIN SIDE UP ON THE LEEKS AND RADISHES.
BRIEFLY TOSS THE TURNIPS WITH 1 TBSP OF BUTTER. ARRANGE ON THE PLATES.
SCATTER WITH CHOPPED PARSLEY OR CHIVES AND GARNISH WITH LEMON SLICES.
SERVES 2