Heat 1 Tbsp. of the oil in a small skillet set over medium heat. Add the mushrooms and garlic and cook until tender, about 2 to 3 minutes. Add the lemon juice, salt and pepper; remove the skillet from the heat. Brush the baguette slices lightly with the remaining olive oil. Broil until lightly toasted on both sides. Spread the bread slices with the goat cheese. Top each bread slice with an equal amount of the mushrooms and a 1/2 slice of the prosciutto. Garnish each crostini with a rosemary sprig and serve.
Options: Instead of prosciutto, top these crostini with thin slices of another Italian-style meat, such as salami or capicollo. These crostini can be topped with other types of cheese, such as thin slices of Brie or cambozola. Rather than rosemary, garnish with small leaves or sprigs of another type of fresh herb, such as basil, oregano, or tarragon.