But for fillings: you might drive yourself mad considering the options. With all tarts and quiches, it's a good idea to put a layer of cheese on the bottom, which seals up any holes. So keep that in mind if you're going to go rogue and use just tomatoes, basil, salt & pepper, or saute chopped cauliflower or broccoli with shallots and garlic and a bit of nutmeg and then pour a light custard over the top, or simply, say, ratatouille and chopped cooked spinach and goat cheese. If you want to copy what I had, follow this modification of Somerville's recipe.
Leek & Olive Tart: Saute 3 thinly sliced leeks in a bit of oil. Add 2 cloves minced garlic, a big pinch of salt, a few grinds of pepper, and cover and steam until softened, then cook uncovered for 1 or 2 minutes until any remaining liquid cooks off. It a crust forms on the bottom of the pan, deglaze it with a bit of wine, stock, or water. Mix this up with a handful of chopped olives and a handful of chopped fresh parsley. After you've dusted the 9-inch tart shell with grated Parmesean (or any other cheese you like), make an even layer of the leek mixture. Whisk together 2 eggs and 1 cup half-and-half (or 1/2 cup cream + 1/2 cup milk), and then pour this over the leeks, and up to the rim of the shell (you may not need to use all the custard). Bake in a 375 degree oven for 25-35 minutes, until fully set.