Preheat your oven to 350 degrees F. Butter a 9-inch springform pan; set aside.

Spread hazelnuts on a baking sheet. Bake until fragrant, about 10 to 15 minutes, then let cool enough to remove the skins.

Place nuts in the bowl of a food processor, and pulse until medium fine, about 30 pulses.

Transfer nuts to the bowl of an electric mixer and add 1 ½ cups flour, ½ cup granulated sugar, brown sugar, ½ teaspoon cinnamon, and ½ teaspoon salt; mix until just combined.

Add butter, and mix on low speed until crumbs begin to stick together, about 2 to 3 minutes.

Press 3 cups of crumb into the bottom of the prepared pan and about 1 ½ inches up the sides of the pan to form crust.

Set remaining crumb mixture aside. Transfer crust to the oven; bake until it appears to be set, 15 to 20 minutes.

Slice plums in half, and remove pits. Slice larger plums into eighths and smaller ones into quarters and arrange in cooled crust.

In a medium bowl, whisk together 1 tablespoon flour and 6 tablespoons granulated sugar.

Whisk in egg, egg yolk, heavy cream, milk, ¼ teaspoon cinnamon, ¼ teaspoon salt, and a few gratings of fresh nutmeg.

Pour custard over fruit; sprinkle with reserved crumb mixture.

Transfer tart to the oven; bake until custard has set and is slightly golden, 45 to 50 minutes.

Let rest at least 25 minutes before cutting if serving warm. You can serve this warm or at room temperature but it is preferred fully chilled.