Spread pastry cream evenly on the bottom of the crust. Arrange fruit in overlapping concentric rows, beginning at outside edge and working in towards the center. For a "fruit salad" tart, toss fruit in a bowl with a drop of Kirsch (optional). Pile the fruit on the pastry cream.
Edge the crust with the nuts and sprinkle on fruit at the edge of the tart. Either brush glaze on the fruit or sprinkle with confectioner's sugar.
For the dough: Place flour, sugar, salt, and baking powder in a food processor fitted with a metal blade. Pulse 3 or 4 times to mix. Add butter and pulse about 12 to 15 times until the mixture is reduced to a fine powder. Do not allow to become pasty. Add egg and liquid, and continue to pulse until the dough forms a ball.
Remove dough from work bowl to a lightly floured work surface and pat into a 5-inch disk. Use immediately, or wrap in plastic and chill until firm before using.
To make a free-form tart crust, roll dough to a 12-inch disk and transfer it to a parchment- or foil-lined cookie sheet, or the back of a jelly-roll pan. Using a plate or cardboard disk as a pattern, trim the dough to an even 12-inch diameter. Fold under 1/2-inch of dough along the edge of the dough, then flute the edge as if it were a pie, to make the edge stand up. Use the thumb and index finger of one hand on the outside of the tart shell and the index finger of the other hand on the inside and pinch gently from the outside while pressing lightly from the inside. Repeat all around the edge of the crust.
Pierce the dough with a fork before baking. Place the dough on the middle rack of a preheated 400 degree oven. Lower temperature to 350 and bake for about 20 minutes. Remove the pan from oven and transfer to a rack to cool. For the Pastry Cream: Combine sugar, flour and salt in a 1 to 1 1/2-quart non-reactive saucepan. Stir with a small whisk to mix. Whisk in milk, then egg and yolk. Place over low heat, whisking constantly, until the pastry cream thickens and comes to a boil. Allow to boil 10 or 15 seconds, whisking constantly.
Remove from heat and whisk in vanilla and butter. Scrape the pastry cream into a non-reactive bowl and press plastic wrap against the surface. Refrigerate until cold, about 1 hour. Stir in liqueur just before using pastry cream. For the glaze: In a small saucepan bring the preserves and water or liqueur to a simmer, stirring often. Strain into another pan and return to a boil. Allow to cool for 5 minutes. NOTE: If tart is mostly red fruit, use currant jelly and strawberry or raspberry preserves.