PEEL THE POTATOES AND CUT INTO ROUGH CHUNKS.
TRIM THE LEEK, LEAVING THE WHITE AND PALE GREEN LEAVES. SLICE LENGTHWISE IN HALF. RINSE UNDER RUNNING WATER TO REMOVE ANY DIRT. SHAKE DRY AND CUT UP ROUGHLY.
TRIM AND PEEL THE ONION. CUT UP ROUGHLY.
MELT 3 TBSPS BUTTER IN A TBSP OF OIL IN A LARGE SAUCEPAN ON A MEDIUM-LOW FLAME.
ADD THE ONION AND COOK UNTIL IT STARTS TO SOFTEN. ADD THE LEEKS AND COOK UNTIL THEY SOFTEN. DUMP IN THE POTATOES AND COOK BRIEFLY.
POUR IN THE STOCK. ADD A FEW SPRIGS OF THYME. SALT AND PEPPER. BRING TO A BOIL, THEN SIMMER FOR ABOUT 15 MINUTES UNTILT THE POTATOES ARE EASILY PIERCED WITH A SHARP KNIFE.
WHEN THE SOUP HAS COOLED, PURÉE WITH AN IMMERSION BLENDER OR IN A FOOD PROCESSOR. CHILL UNTIL READY TO SERVE.
MIX 1/2 CUP OF CRÉME FRAÎCHE WITH 2 TBSPS OF HEAVY CREAM. STIR IN THE CAVIAR.
DIVIDE THE SOUP INTO 6 EQUAL PORTIONS. SPOON A GLOP OF CAVIAR CREAM IN THE CENTER OF EACH BOWL AND SERVE.
OPTIONAL: FRESHLY-CHOPPED CHIVES.