These are so delicious. You can freeze the dough if you are not going to use both logs, you can freeze one for future use.
5 ounces Boursin, at room temperature
½ cup grated Parmesan cheese
¼ cup (1/2 stick) unsalted butter, softened
1 teaspoon kosher salt
¼ teaspoon ground white pepper
1 large egg
2 tablespoons chopped fresh chives
½ cup all-purpose flour, plus more as needed
½ cup white whole-wheat flour
Poppy seeds or chopped nuts for garnish
Put Boursin in the bowl of a stand mixer fitted with a paddle attachment. Add Parmesan, butter, salt and pepper and beat just to combine. Add egg and chives; beat to combine. Add all-purpose flour and whole-wheat flour and mix at low speed, adding more all-purpose flour, 1 tablespoon at a time, until a cohesive dough forms (it should have the feel of soft Play-Doh).nullADVERTISEMENT
divide the dough in half and shape each piece into a 6-inch log, 1 1/2 inches in diameter. Wrap the logs tightly in plastic wrap and freeze until firm, about 1 hour, or refrigerate for up to 3 days.
Preheat oven to 400 degrees F. Line 2 baking sheets with parchment paper.
Spread a layer of seeds or nuts, if using, on a rimmed baking sheet. Roll the chilled logs firmly in the seeds to coat. Slice into even 1/8- to 1/4-inch-thick slices. Arrange about 1 inch apart on the prepared baking sheets.
Bake one pan of crackers at a time, rotating the pan halfway, until light golden around the edges and browned on the bottom, 12 to 16 minutes. Let cool completely on the pan on a wire rack, about 20 minutes.
Place a rack in middle of oven and preheat to 350°F. Spread out walnuts on a rimmed baking sheet and toast, tossing once, until golden brown, 10–12 minutes. Let cool.
Meanwhile, heat a 10″ ovenproof skillet, preferably cast iron, over medium. Add 2 Tbsp. butter and swirl to coat; arrange apple halves cut sides down in a single layer in skillet. Cook apples, undisturbed, rotating skillet on burner as needed for even browning, until cut sides are golden brown, 5–10 minutes (the timing depends on the juiciness of the apples; juicier apples will take longer). Turn apples over and cook on rounded sides just until they start to release their juices and the tip of a knife slides through with slight resistance, about 5 minutes. Transfer apples to a plate, arrange cut sides up, and let cool.
Add ¼ cup brown sugar and 1 Tbsp. water to skillet and set over medium heat. Stir with a wooden spoon or heatproof rubber spatula until sugar is dissolved, then cook, swirling skillet occasionally (do not stir at this point), until thick and bubbling in a thin layer, about 1 minute. Let caramel cool in skillet.
Meanwhile, pulse toasted walnuts, flour, salt, baking soda, baking powder, nutmeg, and 1½ tsp. cinnamon in a food processor until combined and nuts are finely ground. Transfer walnut mixture to a medium bowl.
Combine granulated sugar, remaining ¼ cup brown sugar, and remaining 8 Tbsp. butter in food processor (no need to wash it) and process in long pulses until mixture is light and smooth. Add eggs and vanilla and process in long pulses, scraping down sides of processor once, until mixture is smooth. Add half of walnut mixture and pulse to combine, then add yogurt and pulse just until incorporated. Add remaining walnut mixture and pulse just to combine.
Arrange apple halves cut sides down over cooled caramel in skillet, spacing evenly. Fold batter a few times with spatula, scraping sides to make sure everything is well mixed. Scrape batter over apples and work into spaces around apples. Smooth surface (it’s okay if there’s only a thin layer of batter in places; it will rise in the oven).
Bake cake until it is browned across the entire surface and the center springs back when gently pressed, 30–40 minutes. Let cake cool in skillet 10 minutes, then run an offset spatula or a small knife around the sides of the pan to loosen. Set a wire rack upside down on skillet and flip over to release cake; carefully remove skillet. If any apples or cake stick to skillet, scrape them off and press back onto the top of the cake.
Serve cake warm or room temperature. Slice into wedges and top with dollops of whipped cream and a dusting of cinnamon.
Do Ahead: Cake can be baked 2 days ahead. Let cool completely. Store tightly wrapped at room temperature.