Ideally, a tart crust should be made with a “pastry flour”, but that’s damned hard to find these days. All-purpose will make a tougher crust; cake flour won’t be cohesive enough. I use White Lilly (says “all-purpose”, but it’s actually milled from soft winter wheat), but you may not find that in Canada. Alternatively, you can use ⅔ all-purpose, ⅓ cake.
Working the butter into the flour is a crucial step. You want the flour granules to get coated with fat so the crust will bake up “short” (meaning that it crumbles easily), but you want the occasional little lump of butter in there too, as this will contribute to a layered, flaky structure. These goals are at odds. If you process it only to the point where there are still lumps, then it’s unlikely that the flour has been well coated; if the flour is well-coated, chances are there will be no more lumps. You can get around this by processing in ⅔ of the butter thoroughly (no lumps), then adding the remaining ⅓ and processing it just until the lumps are pea-sized.
The caution against overworking tart dough only applies to mixing after the liquid (water, milk or egg) has been added. This is when you risk over-activating the gluten and making the crust tough. So I mix in the liquid by hand, not in the processor. Transfer the flour/butter mix to a bowl, make a crater in the center of the flour, pour all of the liquid in the crater, then very quickly stir it all together with spatula. The faster that mix gets made, the better. When the whole mess comes together in a lump, stop mixing.
Pat the lump out into a flattened disk, wrap it in plastic wrap and bung it into the fridge. Let it chill there for an hour or so. It will be pretty stiff when you take it out to roll it, so you may need to whack it repeatedly and firmly with the rolling pin to get it supple enough to roll. Roll it on a wood surface.
When it’s the desired thickness, transfer the sheet of dough to a sheet and chill it again before cutting and putting it in the tart molds. Then chill the lined molds before baking, too. All this chilling will keep the dough from shrinking so much during baking, and will keep the crust tender.