Preheat oven to 425F. Line a large baking sheet with parchment paper or silicone mat. Prepare a large pastry bag tipped with a simple, round tip.
Prepare the pastry dough first. In a large saucepan, combine the water, salt and butter. Cook over medium heat until the butter is melted and mixture comes to a simmer. Add all the flour at once and stir with a spatula for about 1 minute; the mixture will come together into a dough ball.
Remove the dough from heat and allow it to cool until just warm to touch, about 10 to 15 minutes. Begin adding the eggs, one at a time, mixing well after each addition. The dough will come together after adding the last egg. Transfer the dough into prepared pastry bag, packing the dough without air pockets.
Pipe the dough onto prepared pan, creating a 9-inch circle. Pipe 5 to 7 rings, some concentric and some overlapping (see video). Sprinkle the top with slice almonds. If making mini Paris-Brest, pipe 4-inch circles. Bake pastry at 425F for 10 minutes, then reduce heat to 350F and bake for 40 to 45 minutes without opening oven door. Pastry should be a rich golden-brown color. Allow the pastry to cool completely before using a serrated knife to split open.
Prepare the cream: in a large mixing bowl, beat the softened mascarpone for about a minute until it's smooth. Mix in the melted white chocolate, if using. Add the chilled heavy cream and confectioner's sugar and whisk again until stiff peaks form, about 4 minutes. Transfer the whipped cream into a pastry bag; I used tip #8FT.
Transfer the lower half of the pastry onto a cake stand, then pipe a generous amount of cream into the center. Top with berries and sliced fruit. Pipe additional cream into the top half of pastry, then cover the first half. Dust the top of the dessert generously with confectioner's sugar. Keep dessert refrigerated.