Preheat oven to 375ºF.
In a medium-size skillet, melt the butter over medium-high heat. Add the leeks, garlic and thyme to the pan, and sauté 5-6 minutes, until very soft.
Sprinkle the flour over the veggies and cook about a minute. Whisk in the stock and bring up to a bubble to thicken the mixture up. Season with salt and freshly ground black pepper and reserve.
While the leeks are cooking, spray the insides of a 6-cup muffin tin with nonstick cooking spray. Line each cup with a slice of ham or prosciutto folded in half.