Step 1

Preheat oven to 425°F, with rack at center

Step 2

Grease a cast-iron skillet with a tablespoon of the butter

Step 3

Listicle: Premium RUST-RESISTANT™ Cast Iron Skillet Circle the potatoes in layers, seasoning with salt, pepper and nutmeg and dotting with pieces of butter as you go

Step 4

Sprinkle in the defrosted chopped spinach as you go, as well

Step 5

Pour over the half-and-half

Step 6

Bring to boil and simmer 10 15 minutes

Step 7

Reduce the half-and-half by half

Step 8

Top with Gruyere and Parm and bake until golden, 10 to 15 minutes