BALSAMIC STEAK & AUBERGINE SALAD

1 red onion • 2 tbsp olive oil • 2 x 200g rump steaks, trimmed of excess fat • About 15 sprays Heinz Balsamic with Garlic Salad Spray • 1 aubergine, thinly sliced lengthways • 1⁄2 x 130g bag watercress, spinach and rocket salad • 160g cherry tomatoes, halved • 20g parmigiano reggiano, shaved