Preheat the oven to 350°F/175°C.
Combine the flour, salt, sugar, and baking powder in a large bowl and whisk to combine. I like to use my dough whisk for the whole mixing process, but a regular whisk would work fine.
In a separate bowl, combine the oil and starter. Whisk to combine.
Pour the combined oil and starter into the flour mixture and stir to combine. This will not come together in a smooth dough, more like a crumbly mixture.
Spread the mixture into a standard 9-inch pie plate, quiche dish, or tart pan. Using your hands, press the mixture flat, covering the bottom of the dish and up the sides. Be firm and try to get it as even as possible to ensure even baking.
If you like, spread 1 tsp of good quality mustard (I like to use dijon because it spreads easily, but a grainy mustard is also very good) over the bottom of the pie crust. Set aside while you prepare the filling.
If you haven't yet steamed your chopped broccolini, do so now, just for a minute or two. You don't want it to get mushy! Grate your cheese of choice as well. I like to use a sharp cheddarish type, like Beecher's Flagship, but almost any cheese would work well - feta, gruyere, comte, parmesan: whatever you have in the fridge.
In a bowl, whisk together the eggs and yogurt until it is completely smooth. Add a pinch of salt and a splash of heavy cream if you have it, and whisk again, thoroughly.
Assembly time! Spread the grated cheese evenly over the bottom of your prepared crust.
Spread the steamed broccolini as evenly as possible over the cheese.
Pour your custard filling over the broccolini, trying to cover most of the pie plate. You may need to pick up the pan and use gravity to help fill in the gaps. Use your hands to help tuck any rogue bits of broccolini under the custard. It won't be totally covered, but you don't want huge chunks sticking up.
Bake for 35-40 minutes. It should be set when you remove it from the oven. Allow to cool for at least 10-15 minutes before cutting in; this is great warm or at room temperature.