Heat barbecue plate and grill on medium-high heat. Sprinkle Cajun spice over both sides of the fish. Spray mushrooms and fish lightly with oil.

Barbecue fish and mushrooms on lightly greased barbecue plate for 3-4 minutes each side, or until just cooked. Spray buns lightly with oil and barbecue on the grill plate, cut-side down until toasted.

Spread tartare or tzatziki over the bun bases. Top with a little of the rocket, followed by tomato, fish and mushrooms. Top with a little more tartare or tzatziki, remaining rocket, and serve with beetroot crisps.