2. Mix the olive oil, garlic, salt, pepper and 1 teaspoon of the basil together. Remove the stems from the mushrooms and brush both sides of the caps with the olive oil mixture. Set aside.
3. Grill the mushrooms until they are soft in the center, about 3 to 4 minutes on each side. Slice each cap on the diagonal into 1" strips.
For the sandwiches:
4. Mix the cheese, the remaining teaspoon of basil, salt and pepper together. Set aside.
5. Spread 6 halves of bread with the cheese mixture, then top with the spinach or arugula, a slice of tomato and several portobello mushroom strips. Place the other half of the bread on top.