Streusel (crumble top) is optional. In a food processor, blend all ingredients until it comes together; refrigerate. Grate streusel using an upside down cooling rack or cut into small cubes. Keep streusel in the refrigerator or freezer until ready to use.
Brioche Dough
Cubed butter and keep refrigerated. In a stand mixer fitted with the paddle attachment (hook attachement works fine too, paddle works faster), mix to combine flour, sugar, salt and instant yeast. Add cold eggs and milk. Mix on low until it comes together. Then, increase mixer speed to medium and mix for 8 mins or until dough comes off the sides of the bowl and becomes elastic. Scrape the sides of bowl and attachment every so often during mixing (it minimizes the factor). Add cubed and cold butter and continue mixing for an additional 8 mins. Scrape the sides of bowl every so often. After that time, the dough should come off easily from the sides of the bowl. The internal temperature should read ≈77/80ºF(25/27ºC). Transfer brioche dough onto the countertop (no extra flour is needed) and shape into a tight round; place dough in a greased container, cover and let ferment for about 90 mins or until it has doubled in size. Transfer dough onto work surface (no extra flour is needed) deflate completely and shape into a tight log. Place over a frozen tray, wrap in plastic film and refrigerate overnight. The resting time in the refrigerator allows the brioche to strengthen and develop aromas.
Shaping
Flour work surface and brioche. Divide dough into three 10oz/300g portions. Deflate dough and shapes into super tight rounds; refrigerate to rest 30 minutes. Roll out each brioche rounds into 6-inch/15 cm disks and refrigerate to rest for 20 mins or so. Save one brioche disk for the recipe and freeze remaining for later use. Roll out brioche into a 9-inch/23cm round and prick dough and press down the edges.
Proofing
Cover brioche loosely with plastic wrap and let proof for about 2:30 hours or until it has largely doubled in size. Gently egg wash brioche and top with some of the frozen streusel.
Baking
Bake brioche at 350ºF/180ºC for about 22 mins. If using a fan oven, cooking time shall be shortened. For better handling, the brioche should be baked a day before slicing. Refrigerate overnight.
Orange Syrup
Bring to a boil all ingredients together but the flavoring. Once at room temp, add flavoring. Use syrup at room temperature.
Diplomate Cream
Soak gelatin in cold water to soften, and drain. Bring milk, a third of the sugar and vanilla to a boil. Meanwhile, combine yolks with sugar and starches. Temper yolk mixture with hot milk. Return mixture to the saucepan, bring to a boil and cook for 2 min whisking constantly. Remove from the heat, add the soften gelatin and mascarpone and then flavoring. Transfer custard onto a frozen baking tray lined with plastic wrap and top with another plastic sheet in contact – refrigerate for up to 2 days. Whip heavy cream to medium-firm peaks (it has to hold its shape) and transfer whipped cream to a pastry bowl and keep refrigerated. No need to clean the bowl – beat chilled custard until smooth. Fold whipped cream into the pastry cream.
Creme diplomate can be piped out straight over the brioche or use a tart ring. If so, seal bottom ring with a tight plastic wrap sheet (humidify the edges with a sponge). Fill tart ring with diplomate cream and freeze overnight.
Montage
Using a bread knife, slice brioche lengthwise and moisturize the top (crust side up) with the orange syrup. Flip and let rest a few minutes. Do like with the bottom brioche. Cut brioche top (crust side down) into desired portions.
Place the frozen diplomate cream wheel over the bottom brioche and arrange slices in order; refrigerate. Finish cutting Tropezienne through when the filling is completely thawed wiping off knife between each cut.
Dust with powdered sugar if desired.
Strawberry Gelée
Give strawberries a quick and gentle wash and lay them over a tray lined with paper towels; check it to pat dry. Remove stems and puree 300g strawberries along with lemon juice. Heat up strawberry puree. Meanwhile, combine sugar and agar-agar. Add it to the strawberry mixture, bring to a boil and cool mixture on ice and refrigerate to set.
Strawberry Compote
When strawberry gelée is completely set, smooth it out using an immersion blender. Cut remaining strawberries into desired sizes and fold into the gelée.