- 2 large leeks (white parts only), chopped
- 1 tablespoon unsalted butter
- 1 tablespoon olive oil
- 3/4 lb cavolo nero or other kale, stems and center ribs discarded and leaves coarsely chopped
- 1 cup chicken broth
- 3/4 cup water
- 2/3 cup heavy cream
- 1 1/2 oz coarsely grated Pecorino Romano (1/2 cup)
- 1 large whole egg
- 2 large egg yolks
- Special equipment: 8 (3- to 4-oz) ramekins (1/3 to 1/2 cup)
- Wash leeks in a bowl of cold water, then lift out and drain well. Cook leeks in butter and oil in a 5- to 6-quart heavy pot over moderate heat, stirring occasionally, until softened, about 6 minutes. Stir in kale and cook, covered, until slightly wilted, about 2 minutes. Add broth and water and simmer, uncovered, until kale is tender, about 8 minutes.
- Purée kale mixture in 2 batches in a blender until very smooth (use caution when blending hot liquids). Pour through a fine-mesh sieve into a bowl, pressing on and discarding solids.
- Preheat oven to 325°F.
- Bring cream and cheese just to a boil, then remove from heat. Steep, covered, 10 minutes.
- Whisk together whole egg, yolks, and kale purée in a bowl. Pour cream mixture through fine-mesh sieve into kale mixture and whisk to combine. Divide among buttered ramekins and bake, uncovered, in a water bath until set, 30 to 35 minutes (centers will still be slightly wobbly).
- Transfer ramekins with tongs to a rack and cool 5 minutes. If desired, invert custards onto plates by first running a thin sharp knife around edge of each ramekin.