1. Preheat the oven to 180°C, gas mark 4. Unroll the pastry, leaving it on its baking parchment and place on a baking tray. Score a 1.5cm wide border around the edge. Brush the pastry with egg wash and prick over the whole area within the border with a fork. Bake for 15 minutes until a light golden brown. Leave to cool.
2. Mix the crème fraîche, eggs, Gruyère and tarragon together and season. Blanch the asparagus in boiling water for 2 minutes, then refresh under cold water. Put in a bowl, season, then toss with the zest and oil.
3. Spread the crème fraîche mixture over the pastry, avoiding the border, then dot over teaspoons of the ricotta. Top with the asparagus. Bake in the oven for 20 minutes until the filling has just set and the asparagus has started to brown.
4. To make the salad, sprinkle the onion with salt and cover with iced water. Leave to soak for 10 minutes, then drain. Arrange the tomatoes and onions on a plate, then season. Drizzle over the olive oil and lemon juice. Garnish with a few basil leaves and serve with the tart.