tablespoons (1 stick) unsalted butter, divided
6 1/2-inch-thick diagonal slices sourdough baguette, or 3 slices sourdough country loaf, halved
tablespoons extra-virgin olive oil
pounds small mushrooms (such as chanterelles, morels, crimini, or button mushrooms), brushed clean (halved if large)
cup heavy cream
fresh oysters, shucked, chopped, drained (optional)
small shallots, minced
tablespoon finely chopped flat-leaf parsley
very thin slices lardo (optional)
Lardo is cured fatback, often seasoned with herbs and spices. It can be found at specialty foods stores and laquercia.us.
Melt 2 Tbsp. butter in a large heavy skillet over medium heat. Add 3 bread slices and toast until golden and crisp, about 3 minutes. Add 1 Tbsp. butter to skillet; turn bread and toast until golden and crisp, about 3 minutes longer. Transfer toasts to a wire rack; let cool. Repeat with remaining 3 bread slices and 3 Tbsp. butter. DO AHEAD: Can be made 1 hour ahead. Let stand at room temperature. Wipe skillet clean. Heat oil in skillet over medium-high heat. Add mushrooms and sauté, tossing frequently, until lightly colored but still firm, about 2 minutes. Season with salt and transfer one third of mushrooms to a plate. Add 2 Tbsp. butter and cream to skillet. Bring to a simmer; cook until cream is very thick and mushrooms are well coated in sauce, 3–4 minutes. Stir in oysters, if using, and shallots and parsley. Divide toasts among plates; spoon mushroom mixture over. Top with reserved mushrooms. Lay 2 slices lardo over each toast, if using. Serve immediately.