STRIP THE STEMS AND THE STRINGS FROM THE SUGAR SNAP PEAS.
GRATE THE ZEST FROM THE LEMON AND PUT IN A BOWL ALONG WITH ITS JUICE.
CUT 6 OZ (3/4 STICK) OF SWEET BUTTER INTO THUMBNAIL-SIZED PIECE AND PLACE ON A SMALL PLATE.
TRIM THE ROOTS AND ALL BUT AN INCH OF STEM FROM THE TURNIPS. SLICE LENGTHWISE IN HALF.
PUT THE TURNIPS IN A FRYING PAN WHICH ACCOMMODATES THEM ALL IN ONE LAYER. POUR IN ENOUGH WATER TO BARELY COVER, SALT AND 2 TBSP OF BUTTER. BRING TO A BOIL, REDUCE TO A SIMMER. COOK FOR ABOUT 10 MINUTES OR UNTIL THEY ARE EASILY PIERCED WITH A SHARP KNIFE.
MEANWHILE...
TRIM ROOTS FROM THE SCALLIONS AND MINCE.
MELT 2 TBSPS OF BUTTER IN A STAINLESS STEEL FRYING PAN LARGE ENOUGH TO HOLD THE COD FILLETS.
ADD 1/2 CUP OF WHITE WINE AND 1/2 CUP OF WATER. ADD THE THYME SPRIGS. SEASON WITH SALT AND WHITE PEPPER. BRING TO A BOIL. REDUCE TO A SIMMER.
LOWER THE COD FILLETS INTO THE SIMMERING LIQUID. COVER AND SIMMER ON A LOW HEAT FOR ABOUT 6 MINUTES, DEPENDING ON THEIR THICKNESS. TEST FOR DONENESS BY INSERTING A SHARP KNIFE IN THE CENTER OF THE FILLET, THEN TOUCHING IT TO YOUR BOTTOM LIP. IF IT'S WARM, THE FISH IS DONE. REMOVE TO TWO WARMED DINNER PLATES.
WHILE THE COD COOKS, HEAT A CAST IRON PAN ON TOP OF THE STOVE FOR A COUPLE OF MINUTES ON A HIGH FLAME. ADD A GLUG OF OLIVE OIL AND THE SUGAR SNAP PEAS. COOK FOR ABOUT 2 MINUTES, SHAKING THE PAN OCCASIONALLY BUT ALLOWING THE PEAS TO CHAR A BIT.
POUR THE LIQUID FROM THE PAN. DISCARD THE THYME. MELT 1 TBSP BUTTER IN THE PAN. ADD THE SCALLIONS AND COOK FOR JUST A MINUTE ON A LOW FLAME.
ADD A SPLASH OF WHITE WINE, THE LEMON JUICE AND ZEST. BRING TO SIMMER, THEN TURN OFF THE HEAT AND BEAT IN THE BUTTER WITH A WOODEN SPOON, A FEW PIECES AT A TIME. YOUR FINGERS SHOULD SOFTEN THE BUTTER SO IT ABSORBS QUICKLY.
DUMP THE CHARRED SUGAR SNAPS INTO THE LEMON BUERRE BLANC.
SURROUND THE COD FILLETS WITH THE TURNIPS AND THE SUGAR SNAP PEAS. NAP THE TOP OF EACH FILLET WITH A SPOONFUL OR TWO OF THE SAUCE.
SERVES 2