THE DAY BEFORE, PUT THE SWEETBREADS IN A BOWL. ADD 2 TBSPS OF SALT AND 4 TBSPS OF WHITE VINEGAR AND COVER WITH WATER. REFRIGERATE OVERNIGHT.
DRAIN THE SWEETBREADS AND PLACE IN A SAUCEPAN. COVER WITH WATER. ADD A FEW TBPS OF SALT. BRING TO A BOIL. REDUCE TO A SIMMER.
POACH THE SWEETBREADS FOR AROUND 15 MINUTES UNTIL THEY ARE SPRINGY BUT NOT HARD. DRAIN IMMEDIATELY AND PLUNGE INTO ICED WATER. WHEN THEY'RE COOL ENOUGH TO HANDLE, PEEL OFF THE OUTER MEMBRANE AND SEPARATE GENTLY INTO LOBES ABOUT THE SIZE OF A WALNUT.
FOLD IN A DISH TOWEL, LAY IN A COLANDER AND WEIGH DOWN THE SWEETBREADS WITH A HEAVY POT FILLED WITH WATER. THIS WILL SQUEEZE OUT MOST OF THE MOISTURE.
BRING A LARGE SAUCEPAN OF WATER TO A BOIL AND SALT GENEROUSLY. MEANWHILE, TRIM OFF THE STEM ENDS OF THE STRING BEANS. BOIL FOR 3-5 MINUTES UNTIL THE BEANS ARE STILL CRUNCHY. DRAIN IN A COLANDER AND IMMEDIATELY BATHE IN COLD RUNNING WATER. ARRANGE ON A DISH TOWEL AND ROLL GENTLY.
FILL UP THE SAUCEPAN WITH WATER AND BRING TO A BOIL. MEANWHILE, USING A VERY SHARP KNIFE, SLICE OFF A SMIDGEN FROM BOTH THE ROOT AND STEM ENDS OF THE PEARL ONIONS. BOIL FOR ABOUT A MINUTE. DUMP INTO A SINKFUL OF COLD WATER. ONCE THE ONIONS COOL, SQUEEZE THEM OUT OF THEIR SKINS AND DRY ON A TOWEL.
MELT 2 TBPS OF BUTTER AND 2 TBSPS OF OIL IN A FRYING PAN. ADD THE ONIONS AND SAUTÉ, SPRINKLING THEM WITH ABOUT A TSP OF SUGAR. ONCE THEY'VE BROWNED, ADD THE 1/2 CUP OF CHICKEN STOCK. SALT AND PEPPER. POACH ON A LOW FLAME FOR ABOUT 7 MINUTES OR UNTIL TENDER.
FOR THE DRESSING, ADD A FEW PINCHES OF SALT AND A GENEROUS GRINDING OF FRESH PEPPER TO YOUR SALAD BOWL. TRIM AND PEEL THE SHALLOT. MINCE AND ADD TO THE BOWL. WHISK IN 3 TBSPS OF CHAMPAGNE VINEGAR. ADD 3 TBSPS OF GRAINY MUSTARD. WHISK INTO THE VINEGAR. WHISK IN ABOUT 3 TBSPS OF WALNUT OIL AND 3 TBSPS OF AVOCADO OIL.
USING A KNIFE WITH A CURVED BLADE, ROUGHLY CHOP THE HAZELNUTS. PLACE IN A TOASTER OVEN AT 300º FOR ABOUT 5 MINUTES OR UNTIL THE NUTS ARE LIGHTLY BROWNED.
SET UP THREE SMALL METAL BOWLS ON YOUR COUNTERTOP. PUT 1 CUP OF FLOUR IN ONE; AN EGG BEATEN WITH A FEW TBSPS OF WATER IN THE MIDDLE BOWL, AND 1 CUP OF BREADCRUMBS OR PANKO IN THE THIRD BOWL. (IF YOU WANT, ADD A TSP OF CHILI FLAKES OR ESPELETTE PEPPER TO THE BREADCRUMBS.)
POUR ABOUT 2 INCHES OF VEGETABLE OIL IN A DEEP-SIDED PAN (CAST IRON IS FINE). HEAT OIL TO 350º.
DIP THE SWEETBREADS FIRST IN FLOUR, SHAKE OFF EXCESS AND DIP IN THE EGG MIXTURE. ROLL INDIVIDUALLY IN THE BREADCRUMBS AND FRY A FEW AT A TIME IN THE HOT OIL. DON'T CROWD THE PAN. WHEN THEY ARE NICELY BROWNED, TAKE THEM OUT AND DRAIN ON PAPER TOWELS.
TOSS THE STRING BEANS, ONIONS, HAZELNUTS AND SWEETBREADS WITH THE DRESSING. SCATTER WITH FRESHLY-CHOPPED TARRAGON.
SERVES 4