Put oven rack in middle position and preheat oven to 375°F.
Toast bread cubes in a shallow baking pan in oven, stirring once, until golden and crisp, 6 to 8 minutes. Meanwhile, melt butter with garlic, salt, and pepper (on the stove or in a microwave). Put mushrooms in an 8-inch square baking dish and toss with garlic butter.
Bake, stirring occasionally, until mushrooms are soft and juicy, 15 to 20 minutes.
Just before serving, toss mushrooms with parsley and croutons.