Combine onion and 2 tablespoons olive oil in large saute pan over medium heat. Add 1/4 teaspoon salt. Cook until onions are soft and golden brown, about 20 minutes.
Meanwhile, toss mushrooms and artichoke hearts with 3 tablespoons olive oil on a medium baking sheet. Roast for 10 to 15 minutes, until everything is golden brown. Reduce oven temperature to 300°F.
Combine caramelized onions, mushrooms, artichoke hearts, and sun dried tomatoes in a large bowl. Set aside.
Cut baguette into 1/4-inch-thick slices. Place slices on a large baking sheet, and drizzle with remaining 2 tablespoons oil. Bake for 15-20 minutes, until crisp but not browned. Set aside.
Combine mascarpone cheese, minced garlic, minced shallots, remaining 1/4 teaspoon salt, and 1/4 teaspoon pepper in a small bowl. Whisk until combined.
Spread toasted baguette slices with the mascarpone mixture, then top with mushroom mixture. Garnish with shaved Parmesan cheese.