Melt the butter in a large skillet over medium heat. Add the garlic and cook 30 seconds. Add mushrooms and cook, stirring occasionally, until the mushrooms are lightly browned, about 3 minutes more.
Add the flour and cook, stirring, for another 2-3 minutes.
Slowly stir in the warm milk and continue stirring until it starts to bubble. Cook stirring occasionally until slightly thickened, about 5 minutes longer.
Taste and add salt, pepper, and lemon juice to taste. Simmer for another minute or two.
Add the shrimp and simmer another 2-3 minutes until heated through.
Ladle over the vol-au-vent shells and sprinkle with freshly chopped parsley if desired.
Shrimp may be swapped with any protein, or left out entirely and replaced with extra mushrooms or other vegetables for a vegetarian versionVol-au-vent shells may be sold frozen, or in the bakery section. They can also be made easily from store bought puff pastry sheets (see this Taste of Home video for a tutorial)