Preparation

• Combine the apples, sugar, vanilla seeds and pod, if using, and 1/2 cup water in a heavy-duty 4-quart saucepan. Cover and simmer over medium heat, stirring occasionally and adding more water 1/4 cup at a time if the apples become dry, until broken down into a chunky sauce, about 30 minutes. Transfer the applesauce to a medium bowl and add the vanilla extract, if using. Let cool and then refrigerate until chilled, about 1-1/2 hours. Remove and discard the vanilla bean pod, if used.
• Position a rack in the center of the oven and heat the oven to 375°F. Toss the oats, rye, and spelt with the 2 oz. melted butter on a large rimmed baking sheet. Spread in an even layer and bake, stirring once, until golden, about 15 minutes. Set aside.
• Melt the remaining 2 oz. butter with the brown sugar in a 12-inch skillet over medium heat, stirring occasionally, until the sugar is completely melted and bubbling slightly, about 2 minutes. Remove from the heat, add the hazelnuts and toasted grains, and toss until clusters form. Spread the clusters on a large rimmed baking sheet and let cool completely.
• In ten 8- to 10-oz. parfait glasses, alternate two 1/2-cup layers each of applesauce and clusters, starting with the applesauce.
• In a large chilled bowl with a hand-held electric mixer, whip the cream on medium-high speed to medium-firm peaks, about 2 minutes. Top the trifles with the cream and serve.

Make Ahead Tips

The oat-rye-spelt clusters can be made up to 1 week ahead and stored at room temperature, and you can assemble the trifles up to 2 hours before serving.