Place mushrooms and garlic cloves on a baking sheet. Sprinkle salt and pepper powder and drizzle extra virgin olive oil all over.
Toss to coat the mushrooms with the oil. Arrange the mushrooms in a single layer.
Preheat the oven to 360 degrees F (180 degrees C). Keep the tray in the middle rack of the oven and roast the mushrooms for 30 minutes. Remove the tray from the oven and let the mushrooms cool down completely.
Switch off the oven. Spread walnuts on a baking tray in a single layer and place in the warm oven for 10 minutes.
Place the grilled mushrooms, roasted garlic, toasted walnuts, olives, capers, white wine vinegar, lime juice, and parsley in a food processor.
Pulse just until combined. Be careful not to over-pulse or the tapenade will get mushy. Check for salt and pepper and add as needed.
Place the tapenade in a serving bowl and garnish with lemon zest and chopped parsley. Serve!