Preheat the oven to 190°C (Gas Mark 5 / 375°F).
Cut the cauliflower into florets, and steam for around 10-15 minutes, or until fairly soft.
Meanwhile, lightly grease an 8 inch springform cake tin. Dust the pastry and surface with flour, and roll it out until it measures around 12 x 12 inches. Lay it over the cake tin, and gently ease it into the corners, pressing gently. If you end up with any tears, just press the pastry back together. Trim off any extreme excess, but leave an inch or two extra around the edge to allow it to shrink a little during baking. Prick the pastry a few times with a fork, then place in the oven to blind bake for around 20 minutes, until golden brown. If the pastry has puffed up at all, just press it back down gently. Trim off any excess pastry, and reduce the heat of the oven to 170°C (Gas Mark 3 / 325°F).
Melt the butter in a saucepan, and add the flour. Cook over a fairly low heat for a minute or so, stirring regularly, to cook the flour. Add the milk a little at a time, stirring until smooth each time and allowing the sauce to thicken up before adding more (it should take a few minutes in total to add all of the milk). Remove from the heat, add the cheeses (holding back a little for sprinkling on top of the tart), and season generously. Mix well, and leave to cool for 5 minutes.
Add the cream to the cheese sauce, and mix well again. In a large jug, lightly beat the eggs. Add about a tablespoon of the cheese sauce to the eggs, and mix thoroughly with a fork to temper the eggs. Repeat a couple more times, then add the remaining sauce and the fresh parsley, and mix thoroughly.
Add the steamed cauliflower to the pastry, and pour over the sauce. Sprinkle the remaining cheese on top, add the halved cherry tomatoes cut-side up, and finish with some more black pepper.
Bake for around 1 hour, or until the egg mixture is set all the way through. If the cheese or pastry is browning too quickly, cover the tart loosely with foil and return it to the oven.
Allow to cool for at least 10 minutes before cutting (it's great served cold too).