Preheat the oven to 400°F. Butter a 9-inch pie plate or shallow baking dish.

Melt 1 tablespoon of the butter in a large skillet over medium-high heat.

Add fish and cook, flipping once, until just cooked through, about 5 minutes.

Transfer to a plate.

Return the skillet to medium-high heat and melt remaining 1/2 tablespoon butter.

Add shallot and garlic, lower heat to medium and cook, stirring often and scraping up any browned bits, until golden, 3 to 4 minutes.

Sprinkle over flour and cook, stirring often, for 1 minute.

Whisk in half-and-half, salt, nutmeg and pepper and simmer until thickened, about 2 minutes.

In a large bowl, gently toss together spinach, 2 tablespoons of the chives and cooked fish, crumbling it into large flakes as you go.

Pour cream sauce over fish mixture and fold together just until combined.

Transfer to the prepared dish and top with bread crumbs and remaining 2 tablespoons chives.

Bake until lightly browned and bubbly, about 15 minutes.