Place bread slices on a cutting board and, using a 3" round cookie cutter, cut out 24 rounds.
Spray mini-muffin pans with non-stick cooking spray or brush with melted butter.
Press bread rounds into mini-muffin cups.
Place mini-muffin in the pre-heated oven and bake for 10 minutes.
Remove from oven and allow to cool.
Place 4 tbsp. butter in a small saucepan, over medium heat, and cook until melted.
In a medium skillet, over medium heat, cook shallots in the melted butter.
Stir in the mushrooms; cook for 10 to 15 minutes or until moisture has evaporated.
Sprinkle flour over the mushroom and butter mixture; stir to thoroughly combine.
Add cream; bring to a boil while stirring constantly.
Reduce heat and simmer just a couple minutes.
Remove from the heat; stir in salt, pepper, parsley, chives and lemon juice.
Place mixture in a bowl; allow to cool; if not baking and serving immediately, see notes below.
When ready to bake, pre-heat the oven to 350°F.
Place shells on a baking sheet; fill with the mushroom mixture.
Sprinkle the croustades with cheese and dot with butter.
Place in the pre-heated oven and bake for 10 minutes; remove from the oven; set aside.
When ready to serve, turn broiler on high and broil for 10 - 15 seconds.
Notes
These may be placed in an air-tight container and frozen. When ready to bake and serve, remove from the freezer; pre-heat to 350°F; place in the oven and bake for 10 minutes; then place under the broiler for 10 - 15 seconds.