Prepare the Mushrooms: Heat olive oil in a skillet over medium heat. Add the sliced portabella mushrooms and garlic. Sauté for 5-7 minutes until the mushrooms are tender and slightly golden. Season with salt and pepper to taste.
Assemble the Salad: In a large bowl, combine fresh spinach, red onion, and sautéed mushrooms. Toss gently to mix.
Add the Dressing: Drizzle balsamic vinegar over the salad and toss again to ensure even coating.
Garnish: Sprinkle crumbled feta cheese over the top for a creamy, tangy finish.
Serve: Divide into individual bowls or serve family-style on a platter. Enjoy immediately for the freshest taste!