1) In a large bowl, mix together flour, salt, and sugar until well combined. Using clean hands, add butter and toss until just coated. Rub butter between thumb and forefingers to incorporate into flour mixture until butter is in broad bean-sized pieces and comes together in 10p sized clumps when squeezed in palm, about 2 minutes.
2) Add vegetable fat, toss until just coated in flour, then rub into flour mixture until mixture forms pea-size pieces (some big chunks should remain) and comes together in fist-sized clumps when squeezed, about 1 minute.
3) Drizzle in half ice water and rake through mixture with fingers until just moistened. Drizzle in remaining water 1 tbsp at a time and comb through mixture with fingers to moisten. It will go from being a shaggy mess to coming together. Dough is moist enough when it is moistened through but is not wet when pressed. (Do not overwork the dough or it will become tough.)
4) While rotating the bowl with 1 hand, push dough between other palm and side of bowl to gather into a ball. Turn dough onto a piece of cling film, press it into a flat disk, then close in cling film.
5) Place in coldest part of refrigerator (usually back bottom shelf) at least 30 minutes before rolling out and forming into a crust.