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1
Cut off the stem of each mushroom, leaving only the cap (portobellos don't usually have much stem, anyway). Using a spoon, scoop out the black ribs from the inside of the mushroom cap.
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2
Cut the zucchini lengthwise, into 1/2 inch thick slices. Brush the mushrooms and zucchini on both sides with the olive oil.
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3
Heat a grill to high. Grill the zucchini until soft and pliable but not mushy; then remove from the grill and set aside. Sprinkle the mushrooms with salt and grill for 2 to 3 minutes on each side, until browned on the edges and very soft in the middle. While those cook, finish the tapenade filling.
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4
Add the zucchini and walnuts to the bowl of a food processor and pulse until smooth and creamy. Add the capers, anchovies, thyme, oregano, vinegar, and lemon zest and process, pulsing, until just blended.
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5
Add all of the olives and pulse until the mixture is well combined but still contains fairly coarse chunks of olives.
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6
Place the mushroom on a serving plate, hollow side up. Fill each with the tapenade. (You can use as much or as little as you like.) Cover with another mushroom. Cut in quarters, or for smaller mushrooms, in half, and serve. Garnish with additional chopped walnuts, if desired, by rolling the edge in the nuts.
