How to Make It
In a small bowl, whisk 3 tablespoons of the balsamic vinegar with 3 tablespoons of the olive oil, the garlic and a pinch each of salt and pepper. Pour the vinaigrette over the mushroom caps and let marinate at room temperature for 1 hour.
Light a grill or heat a lightly oiled grill pan or cast-iron skillet. Drizzle the cut sides of the tomatoes with 1 tablespoon of the olive oil and season with a pinch each of salt and pepper. Grill the tomatoes over high heat, skin sides down, for about 3 minutes, or until softened and lightly charred. Transfer to a plate.
Grill the mushrooms, turning once, for about 5 minutes per side, or until tender and charred. Transfer to paper towels to drain. Slice each mushroom cap into 4 to 5 pieces.
Brush the cut sides of the baguette with 1 tablespoon of the olive oil and grill or toast until lightly browned. In a small bowl, combine the ricotta with a large pinch each of salt and pepper. Spread the ricotta on the bottom half of the grilled baguette and layer the tomatoes, arugula and Portobellos on top. Drizzle with the remaining 1 tablespoon olive oil and 1 tablespoon balsamic vinegar and cover with the top half of the baguette. Cut into 8 pieces and serve.