The first two steps will take about a half-hour, but need no constant attention. In a small saucepan over low heat, bring the whipped cream to a boil and reduce by half. (be careful that the pan doesn't foam over, as cream has a way of doing.)
Into a second small saucepan, strain the oyster water. Add the cracked peppercorns and thyme. Bring it to a light boil over low heat and reduce to about 1/4 cup of liquid. Strain.
In a skillet, heat the butter over medium heat until it bubbles. Add the oysters, mushrooms, and bacon. Cook until the mushrooms are soft--about two minutes.
Add the reduced cream and the reduced oyster water to the skillet, along with the all but about 1/4 cup of the green onions, plus the parsley and lemon peel. Stir lightly until combined and cook over medium heat about another minute
Using a slotted spoon, spoon the oyster mixture into the vol-au-vents on a baking sheet or pan. This will leave a lot of sauce in the pan. Bake the vol-au vents for twelve minutes.
Meanwhile, bring the sauce in the pan to a light simmer. When the vol-au-vents are ready, remove them from the oven and add a teaspoon or two of the sauce. Top with a sprinkling of green onion and serve immediately.