Special equipment: a 9-inch tart pan
Place a rack in the center of the oven and preheat the oven to 350 degrees F.
For the tart shell: In the bowl of a food processor, pulse the walnuts, rosemary and 2 tablespoons of the flour until the nuts are finely ground. Add the granulated sugar, salt and remaining 3/4 cup flour and pulse to blend. Add the butter and pulse until the dough resembles coarse sand. Pour in 1 tablespoon cold water and pulse until the dough just comes together in a ball. Shape the dough into a disc, wrap in plastic wrap and refrigerate until the dough is firm, about 30 minutes.
Place the dough between 2 sheets of parchment paper and roll it out until it is 1/4 inch thick and 10 inches in diameter. Transfer the dough to a 9-inch tart pan and press the dough into the bottom and up the sides of the pan, trimming away any excess dough. Prick the dough with a fork, place the tart pan on a baking sheet and bake until light brown, about 20 minutes. Set aside while you make the filling.
For the tart filling: Melt the butter in a small saucepan over medium heat until it is golden brown and fragrant, about 3 minutes. In a large bowl, whisk together the brown sugar, honey, cream, vanilla and salt. Add the browned butter, walnuts and rosemary and stir to combine.
Pour the filling into the tart shell and place on a baking sheet. Bake until the filling is bubbly and thick, about 35 minutes. If the edges of the crust begin to get too dark, cover the tart loosely with foil. Transfer the tart to a wire rack to cool.
For the orange chantilly cream: In the bowl of a stand mixer with a whisk attachment, combine the cream, granulated sugar and orange zest and juice and beat until soft peaks form, about 2 minutes.
To serve, dust the tart with powdered sugar and top each slice with a dollop of the cream.