Sourdough Toasts with Mushrooms and Oysters

Name: Sourdough Toasts with Mushrooms and Oysters
Ingredients: 8 tablespoons (1 stick) unsalted butter, divided; 6 1/2-inch-thick diagonal slices sourdough baguette, or 3 slices sourdough country loaf, halved; 2 tablespoons extra-virgin olive oil; 1 1/2 pounds small mushrooms (such as chanterelles, morels, crimini, or button mushrooms), brushed clean (halved if large); Kosher salt; 1/3 cup heavy cream; 12 fresh oysters, shucked, chopped, drained (optional); 2 small shallots, minced; 1 tablespoon finely chopped flat-leaf parsley; 12 very thin slices lardo (optional)