Cut celery root into chunks, boil in salted water for 8 minutes until tender, then drain and blend with 2 tbsp butter, minced garlic, and heavy cream until smooth.
Pat cod fillets dry, rub both sides generously with blackening spice.
Heat 2 tbsp butter in a skillet over high heat, then sear cod for 3 minutes per side until cooked through and dark crust forms.
Plate celery root puree, top with cod, squeeze lemon juice over fish, and serve.