Prepare the Rosemary Infusion: In a small saucepan, heat the olive oil over low heat. Add the fresh rosemary sprigs, smashed garlic clove, and black pepper. Let it gently infuse for about 10-15 minutes. Remove from heat and set aside.
Prepare the Caramelized Onions: Add all ingredients to a dutch oven (enamel-coated cast iron is best), cover, and sweat over medium heat until onions are translucent. Stir occasionally until onions have turned deep golden brown. Once onions have begun sticking to bottom of pan, add a small amount of water or broth and stir to release onions. Once onions have broken down and fully caramelized, remove from heat, transfer to glass storage container(s) and set aside.
Prepare the Mushroom Topping: Heat a large, dry pan over medium heat. You will add oil later but mushrooms should be prepared dry to properly release their natural moisture. Once the pan is hot, add the mixed mushrooms and cook until they become golden brown. Season with salt and black pepper. Add the garlic and a drizzle of olive oil, saute until fragrant. Drizzle truffle oil over the mushrooms and toss to combine. Remove from heat.
Assemble the Crostini: Brush each toasted bread slice with the prepared rosemary-infused olive oil.
Top the Mushroom Mixture: Spoon 1 teaspoon of the sautéed mushroom mixture onto each crostini. Top with ½ tsp Caramelized Onions.
Garnish: and sprinkle freshly chopped parsley over the mushroom-topped crostini for a burst of color and freshness.
Serve: Arrange the Truffled Mushroom Crostini with Rosemary Infusion on a serving platter.
