Preheat your grill to medium-high heat.
Clean the Portobello mushrooms with a damp cloth, removing any dirt or debris. Remove the stems and set the caps aside.
In a mixing bowl, whisk together olive oil, balsamic vinegar, minced garlic, salt, and black pepper.
Brush the mushrooms with this mixture, ensuring they are well coated on both sides.
Place the mushrooms cap side down on the grill. Grill for about 5-7 minutes on each side or until they are tender and have grill marks.
While the mushrooms are grilling, slice the cherry tomatoes in half.
Once mushrooms are grilled, slice them thinly.
In a serving bowl, lay down the baby spinach or arugula, followed by the sliced mushrooms and cherry tomatoes.
Drizzle the remaining olive oil and balsamic mixture over the salad.
Garnish with fresh basil and optional cheese.