Put the broccoli into a heatproof bowl with 2 tablespoons of water. Cover it with cling film and pierce several times. Microwave on high for 3 minutes. Arrange the cabbage, fennel and rocket on 2 serving plates.
Heat a non-stick pan and dry-fry the seeds for a few minutes, turning as needed. Add the lemon juice, zest and olive oil to the pan and whisk to combine and warm through.
Top the crunchy leaves with the drained tuna, then add the broccoli and drizzle over the warm dressing. Season with freshly ground black pepper to taste and serve.