Preheat the oven to 180°C/gas 4. Lightly grease a 24cm round tart tin, 3cm deep, then line with greaseproof paper. Melt the butter in a frying pan, then sauté the spring onion and garlic over a gentle heat to soften.
Snap off the woody ends of the asparagus, then slice into 1cm pieces, keeping the tips whole. Add the asparagus to the pan and cook gently for a couple of minutes. Season with salt and pepper, take off the heat, drain and set aside to cool. Keep the asparagus tips to one side.
Meanwhile, in a bowl, beat the eggs, ricotta, milk and two-thirds of the Parmesan, then season.
Add the cooled asparagus mixture, along with the chicken. Gently stir it together, then pour into the tin, making sure all the ingredients are evenly distributed. Scatter the tips on top of the tart, and push them into the mixture a little.
Sprinkle the rest of the Parmesan on top, then put the tart on a baking tray in the oven for 20–25 minutes, until just about set in the middle. If it’s not golden, put it under the grill for a few seconds.
Leave the tart to cool to room temperature, then garnish with some extra sliced spring onion, cut into slices, and serve.