Lightly butter six x 10 cm diameter tartlet pans and chill.
Place flour and salt in a food processor and aerate with a couple of quick on/off pulses.
Add butter and process until mixture resembles fine breadcrumbs.
Add egg yolk mixture and process until pastry just draws together. (Or rub butter into sifted flour with your fingertips, as lightly as possible, then add egg yolk mixture and pinch into a dough.)
Turn out onto a lightly floured work surface and knead briefly to form a flat round.
Roll out pastry to 2 mm thick and cut out rounds to required size.
Line pans, trim edges generously (pastry might shrink) and refrigerate for at least one hour.
Preheat oven to 190 °C.
Fill moulds with caramelised onion and top with sliced cheese.
Beat remaining ingredients until combined.
Divide among tartlets and bake until bases are golden brown and filling has set, about 10 minutes. Cool slightly.
TOMATO AND BASIL SALAD:
Toss ingredients in a bowl.
Top tartlets with salad and serve.
Instead of six small tartlets, make one 24 cm tart and serve sliced.