Mix the olive oil, mustard, honey and zest and juice of 1½ lemons together in a bowl and season.
Heat the butter and olive oil in a large frying pan. Add the mushrooms and stir well to coat. Sauté for 5-8 minutes or until tender.
Stir in the tomatoes and goat's cheese and warm through for 1-2 minutes. Add the reserved lemon juice and zest. Put the salad leaves and spinach in a large serving bowl, then tip the mushroom, tomato and cheese mixture over the top. Add the dressing and toss everything together. Sprinkle with the toasted pine nuts and serve immediately.