1 Preheat the oven to 220°C (200° fan) | 425F | gas 7.
2 Wash and drain the green asparagus, cut the tips off the spears and reserve.
3 Heat the butter in a pan and sweat the onion until soft. Add the stock, herbs and seasonings and bring to the boil. Reserve 1 tbsp peas and add the rest of the peas, the asparagus stalks (without the tips) and the diced potato to the onion stock. Cook for about 5 minutes. Drain and roughly mash with a potato masher.
4 Mix in the crème fraîche and season to taste with salt, pepper and lemon juice.
5 Place the baguette slices on a baking tray and toast in a hot oven for about 3-5 minutes. Meanwhile quickly sauté the asparagus tips and reserved peas in a little olive oil.
6 To serve, spread the toasted bread while still hot with the pea and asparagus puree. Top with the fried asparagus tips and peas and scatter the crostini with Parmesan shavings.
7 Drizzle with olive oil and sprinkle with pepper if you wish. Arrange attractively on a platter with the watercress.