For the tapenade: In a food processor, combine the olives, red peppers, raisins, capers, parsley, orange zest, tarragon and black pepper. Pulse six or seven times, just until roughly chopped. Pour in the olive oil and pulse 2 or 3 times to incorporate; the mixture should have the consistency of a coarse puree. Cover and refrigerate until ready to use.
For the cauliflower steaks: Preheat a grill or grill pan to medium heat.
Remove the green outer leaves from the cauliflower heads, leaving the cores intact. Set the cauliflowers on their base and halve them vertically (a serrated knife works well for this). Then slice each half, vertically again, into 1-inch steaks; the core will hold them together. You should end up with four nice steaks; reserve the ends for another use.
Generously coat the steaks on both sides with the olive oil and season with salt and pepper. Grill the steaks for 5 minutes per side, or until golden brown and well-marked. Remove to a foil-lined baking sheet.
In a small bowl, mix together the Parmesan, panko, parsley, garlic and pepper flakes. Season with salt and pepper. Sprinkle the breading on top of the cauliflower steaks and drizzle with some olive oil. Transfer to the oven for 10 to 12 minutes, or until golden brown. Serve the steaks topped with spoonfuls of the tapenade.