Blind bake the crust at 400F for 15 minutes with pie weights, then 5 more minutes without. Reduce oven to 375F.
Toss cauliflower florets with olive oil, salt, and pepper. Roast at 400F for 15-18 minutes until golden-brown.
Poach the cod in simmering water for 6-8 minutes until it flakes easily. Drain and flake into large pieces.
Whisk together the eggs, cream, milk, salt, pepper, and nutmeg until combined.
Scatter half the Gruyere in the crust, layer in the cauliflower and cod, top with chives and remaining cheese. Pour the custard over everything.
Bake at 375F for 35-40 minutes until set with a slight jiggle in the center. Rest for at least 15 minutes before slicing.