To make tart crust, pulse flour and salt in a food processor to combine. Add butter and pulse until mixture resembles coarse meal with a few pea-size pieces of butter remaining. Transfer mixture to a large bowl; drizzle with lemon juice and add sour cream. Mix with a fork, add ice water until dough comes together. You might not need all 5 tablespoons. Lightly knead until no dry spots remain and dough is smooth. Pat into a flat 1/2 inch thick disk and wrap in plastic. Chill at least 1 hour.

To make filling, cut 1/3 of brie cheese into 1/4 inch cubes and place in a medium bowl. Thinly slice remaining brie and refrigerate until ready to use.

Add ricotta, thyme, mustard, egg, salt and pepper to brie in the bowl. Stir to mix well.

Preheat oven to 350 F. Butter an 11 inch round loose-bottom tart pan.

Remove dough from refrigerator and let rest to soften for a few minutes. Dust clean surface with flour and roll out dough into a 12 inch round with even thickness. Large enough to fit inside the tart pan. No worries if it over laps, just tuck it in a bit as it will give it a rustic look when baked.

With a fork, prick 10 times on the bottom of pastry. This will help tart not bubble on the bottom when baked. Bake tart shell in oven for 10-12 minutes or until base is dry and cooked. Let cool completely before filling.

To assemble tart, fill cooled tart shell with cheese filling spreading it evenly. Arrange brie slices on top and brush the top with a 1/4 cup of the orange marmalade. Bake tart for 30 minutes

Remove from oven and layer apple slices on top. Brush the apples with the remaining 2 tbsp of marmalade.

Bake tart for an additional 10 mins or until apples are golden caramelized.

Garnish tart with fresh thyme sprigs and serve warm.