Combine balsamic vinegar and soy sauce in a skillet.

Heat the mixture over medium heat.

Add the sliced portobello mushrooms to the skillet.

Poach the mushrooms in the sauce, allowing the sauce to reduce and thicken.

Remove the skillet from the heat.

Let the mushrooms cool down.

In a mixing bowl, combine the cooled mushrooms, diced red onion, baby spinach, and chopped tomatoes.

Toss the salad ingredients together gently.

Serve the salad chilled, family style.

Garnish with minced fresh dill before serving.