Step 1 Combine balsamic, garlic and mustard into a non-metallic bowl. Add steaks, stir to coat, cover and set aside for 20 minutes.
Step 2 Preheat oven to 160ºC. Line a baking tray with baking paper. Place tomatoes, cut side up, onto tray. Roast for 10-12 minutes, or until wilted. Set aside to cool slightly. Combine with parsley, pine nuts and extra balsamic in a bowl. Season with black pepper.
Step 3 Cook potatoes in a saucepan of boiling water for 12-15 minutes until tender. Drain, then return to saucepan with milk. Mash until smooth. Stir through extra mustard.
Step 4 Spray a non-stick frying pan with oil and place over high heat. Cook steaks for 2-3 minutes each side for medium, or until cooked to your liking. Remove, cover loosely with foil and set aside to rest for 2-3 minutes. Serve steak on mustard mash with tomato relish.