To make the dressing, heat the oil in a large non-stick frying pan over medium heat. Sauté the onion for 1 minute. Stir in the tomato juice, lemon juice, mustard, garlic, salt and freshly ground black pepper. Heat until just boiling, then remove from the heat.
Pour the hot dressing over the spinach and toss to coat. Top the salad with the bacon and croutons. Serve warm.
Per serving: 170 calories, 7 g protein, 12 g fat (2 g saturated fat), 9 g carbohydrate (3 g sugars), 2 g fibre, 620 mg sodium