Heat oven to200C/180C fan/gas6. Line a 21 or 22cm square, shallow baking tin with baking parchment. Put the flour, ground rice and sugar in a bowl with the butter and rub in until fine crumbs form. Stir in the milk with a cutlery knife. Tip into the tin and press down evenly. Bake for 15-20 mins until golden.
Remove the tin and lower oven to 180C/160C fan/gas 4. Mix the lemon juice and eggs, then sieve into a bowl with the sugar, flour and zest. Whisk to combine, pour over the base, then bake for 10-15 mins until the top is just set. Cool in the tin, dust with icing sugar, then slice.