Roast mushroom: Heat oven to 425degrees. Brush both sides of mushroom with 1 tsp oil, season with salt and pepper, and place ribbed-side up on a baking sheet. Roast until tender, 18 to 20 minutes. Slice when cool.
Drizzle the toasted baguette with remaining oil. Top one slice of the baguette with mushroom, spinach, and tofu. Top with the second slice.