In a large skillet, bring the stock to a gentle simmer over low heat. Layer the turkey in the skillet and gently reheat to bring the moisture back to the meat.
In another large skillet, heat the oil, one turn of the pan, over medium. Add the garlic and stir until fragrant, about 1 minute. Add the spinach and cook, stirring often, until wilted, about 5 minutes; season with salt and pepper. Remove from the heat.
Toast the crumpets; divide among 4 plates.
For the Mornay sauce, in a medium saucepan, melt the butter. 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, using a fork, beat in the egg yolk, then whisk the mixture back into the sauce in the pan. Add the cheese and, using a spoon, stir until melted, about 2 minutes. Remove from the heat.
Pile the greens on the crumpets and top with sliced turkey. Top with the Mornay sauce and garnish with paprika and chives.