Aioli- Combine Mayo, 3 garlic cloves, lemon juice, 1 TBSP balsamic vinegar and chopped rosemary in small bowl to blend. Season to taste with salt and pepper. (Aioli can be prepared 1 day ahead)
Mix vinegar, olive oil and garlic in a gallon sized zip-lock and place mushroom slices inside. Let marinate for 30 minutes. Preheat BBQ. Remove portobello slices from marinate and grill until soft and cooked through- approx 2 to 5 min per side.
Preheat oven to 375°F. Place baguette slices on baking sheet. Brush with olive oil. Bake until lightly toasted, about 10 minutes.
Top with grilled portobello mushroom slices. Season to taste with salt and pepper. Drizzle bruschetta with garlic aioli. Serve immediately.