In a large skillet, bring the stock to a gentle simmer over low heat. Layer the turkey in the skillet and gently reheat in the stock to bring the moisture back to the meat.
 
In another large skillet, heat the oil, one turn of the pan, over medium heat. Add the garlic and stir until fragrant, about 1 minute. Add the spinach or chard and cook, stirring often, until wilted, about 5 minutes; season with salt and pepper. Remove from heat.

 
Toast the crumpets or muffins; divide them among 4 plates.
 
For the Mornay sauce, melt the butter in a medium saucepan. Add the flour and whisk to blend. Whisk in the milk and season with the dry mustard, white pepper, nutmeg and salt. Cook, stirring often, until slightly thickened, about 3 minutes. If using the egg yolk, scoop a ladleful of the sauce into a small bowl. Quickly, beat in the egg yolk with a fork to temper then whisk the mixture back into the sauce in the pan. Add the cheese and stir with a spoon until melted, about 2 minutes. Remove from the heat.
 
Pile the greens on the crumpets and top with sliced turkey. Ladle the Mornay sauce over the top and garnish with paprika and chives.

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