Blend ingredients 1 through 5 in a food processor and pulse until the dough clumps. If the mixture is very dry and crumbly even after pulsing, add a little sherry or water, just one tablespoon at a time. Dough will be soft (depending on how cold the butter was to start). Chill dough 30-60 minutes, then roll between sheets of wax paper to fit tart pan.

Preheat oven to 400ºF. Line bottom, but not sides, of tart pan with parchment paper. Place dough in pan, patch any holes and push dough along the edges. Trim with knife for tidy edge. If the dough is very cold, it may be quite stiff and you may want it slightly malleable. Dock the crust with a fork and bake in oven until the dough is just barely golden, about 10 minutes.

Beat all remaining ingredients except lemon and apples with beaters or in a food processor. Pour into pre-baked tart pan.

Carefully slice apples in thin crescents (dip in lemon water, draining excess water, if desired). Place on top of custard, overlapping apple slices carefully.

Bake at 400ºF for 20-30 minutes, until filling has puffed up slighted and is perhaps slightly cracked. The deeper the custard layer, the longer the baking time. Remove from oven and cool before removing from tart pans.

Decorate with slivered almonds.