Chef John's Fisherman's Pie

Ingredients

• 12 ounces baby spinach leaves 
• 1 tablespoon butter 
• 1 pinch cayenne pepper to taste
• 2 pounds filets boneless
• 3 tablespoons flour all-purpose
• 1 tablespoon chives fresh chopped for garnish
• 2 cloves garlic minced
• 6 servings salt and ground pepper black to taste
• 0.5  juice of lemon juiced
• 2 teaspoons lemon zest 
• 2 cups milk cold divided
• 1 pinch nutmeg freshly grated
• 2 teaspoons olive oil 
• 3  russet potatoes peeled cut into chunks

Equipment

• bowl
• frying pan
• baking sheet
• paper towels
• sauce pan
• oven
• whisk
• casserole dish
• broiler
• dutch oven

Directions

1. Bring a large saucepan of salted water and to a boil; add russet potatoes to boiling water and cook until very tender, about 20 minutes.
2. Drain well. Mash in 3 tablespoons butter until thoroughly combined. Season with nutmeg, salt, black pepper, and cayenne pepper to taste. Mash 1/2 cup milk into potato mixture until smooth.
3. Drizzle olive oil in a large Dutch oven over medium-high heat, add spinach, and season with a big pinch of salt. Cook, stirring occasionally, until spinach has wilted, about 1 minute.
4. Transfer to a bowl lined with paper towels to wick away excess moisture.
5. Heat 3 tablespoons butter and flour in a saucepan over medium heat; whisk mixture to a smooth paste. Cook, stirring constantly, until mixture has a nutty smell and is slightly browned, about 2 minutes.
6. Add chopped garlic; whisk until fragrant, 10 to 20 seconds.
7. Whisk 1 cup cold milk into flour mixture; cook until thickened.
8. Whisk in remaining 1 cup milk and lemon zest. Bring white sauce to a gentle simmer, whisking constantly; season with salt. Turn heat to very low and keep sauce warm.
9. Preheat oven to 375 degrees F (190 degrees C). Grease an 8x12-inch casserole dish with 1 tablespoon butter.
10. Season buttered pan with salt, black pepper, and cayenne pepper.
11. Lay boneless cod fillets into the pan in a single layer. Season tops of fillets with more salt, black pepper, and cayenne pepper.
12. Spread spinach evenly over fish and drizzle with lemon juice. Spoon white sauce over spinach; give casserole dish several taps and shakes to eliminate bubbles.
13. Drop mashed potatoes by heaping spoonfuls over the casserole and spread smoothly to cover.
14. Place dish onto a rimmed baking sheet to catch spills.
15. Bake in the preheated oven until bubbling, about 40 minutes. Turn on oven's broiler and broil until potato crust has a golden brown top, about 2 minutes. Fish should flake easily.
16. Let stand 10 minutes before serving.
17. Garnish with a sprinkle of chives.