Herein, my opinion of the difference between a tart and a pie: it’s all in the crust.
Pie crust is a cracker – a flaky, tender cracker but a nearly flavorless cracker nonetheless. Its entire purpose is to hold the filling, and it succeeds best by being not – not hard, not tough, not sweet, not tart, not soggy.
Tart crust is a cookie – with far less sugar if you’re making savory tarts, but a firm, flavorful, crumbly cookie nonetheless.
For pie dough I’ve come to appreciate the 3-2-1 method. By weight, 3 parts flour, 2 parts fat, 1 part liquid. As Alton Brown told us all in The Crust Never Sleeps, shortening (or lard) makes tender and butter makes flaky. never mind his mix, I like roughly half of each for my ‘2 parts’. 9 ounces of flour (3 butter, 3 shortening, 3 water) works for me when making a 9 inch slightly deep dish pie.
Just for a reminder, cut cold fat into cold flour till it looks like cornmeal, then sprinkle in the cold water and stir just till it comes together in a dough ball. Remove, put in a closed container and into the refrigerator to let the dough hydrate – 10 to 20 minutes works, half an hour is usually recommended. Put the ball on a lightly floured surface, roll it out, put it in the pan. Whether you bake and then add filling or add filling or then bake depends on what you’re making, but you’ve got the crust at that point.
Now for tart crusts, if you’re really pressed just buy sugar cookie dough. Yep, makes a very tasty tart base (for sweet tarts – I’ll get to savory in a bit).
But if you’re wanting what I think is a better tart crust, you go to shortbread instead – still ‘cookie’, but better. It’s just as easy by the way. 3-2-1, flour-fat-liquid. It’s just a couple of details change. 3 parts flour, still. 2 parts butter, no shortening. (well, you can, but bleah). And for the liquid, 1 part sugar.
Yes, sugar for the liquid. And make this in reverse. Cream the sugar into the butter. This’ll break up the butter so it doesn’t make all those striations (flakiness) in the flour. Mix the flour into this, and as long as the butter remains cold the water won’t separate and you’ll get very little gluten development (and toughness). Into the refrigerator to hydrate and cool, bring it out and roll just like the pie dough.
This shortbread isn’t all that sweet, either, which means it’ll work for a lot of savory tarts. But sometimes you don’t want sweet at all. So what’re you going to do?
Still 3-2-1. Still 3 flour and 2 butter, but this time the liquid is … an egg. An egg is 2 to 3 ounces by weight, so you’ve got a minimum size (or you need to split the resulting dough between pans) for the butter and flour. Mix the butter and flour as for a pie dough. Add the egg and don’t worry about the gluten, just mix till it forms a ball. If the dough’s still too dry and won’t form a ball add a little cold water, about a teaspoon at a time, and mix briefly to see if it comes together. The egg, between its fats and liquids and various nifty proteins, will make the crust short without the sweet. Same drill with the refrigerator and the rolling, of course.
Oh, and all of that above about the chef and such telling you to add stuff? Well, there is nothing saying you can’t add a bit to any of the crusts above. Make a lemon tart, for example, and when you’re making that crust add a bit of lemon zest. There’s no problem putting a minced clove of garlic in the savory crust, either. I’m not convinced of the use of adding anything to the pie crust (the cracker) but I’m not against it, either.