Blondie Sundaes with Fried Walnuts and Candied Fennel

Preheat oven to 350°. Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 5–8 minutes. Let cool slightly; place in a heatproof jar or airtight container.

Heat sugar and 1 cup water in a medium saucepan over medium, stirring, until sugar dissolves. Let cool slightly; then pour over walnuts. Seal jar and let sit at least 12 hours and up to 1 day.

Pour walnuts into a fine-mesh sieve set over a clean medium saucepan and let syrup drain into saucepan, tossing walnuts to shake off excess. Transfer walnuts to a medium bowl. Reserve sieve.

Add sliced fennel and lemon zest to syrup; scrape in vanilla seeds and add pod. Bring to a simmer over low heat. Cook, stirring occasionally, until fennel is translucent and tender, 20–25 minutes. Discard pod; let fennel cool in syrup.

Fit another medium saucepan with thermometer; pour in oil to come 2" up sides of pan. Heat oil over medium-high until thermometer registers 350°. Add powdered sugar to walnuts and toss to coat. Place in reserved sieve and shake off excess sugar. Carefully add walnuts to oil and cook, turning with a spider or slotted spoon to prevent sticking, until brown and crisp, about 3 minutes. Transfer to a parchment-lined rimmed baking sheet; let cool. Break up any clumps.

Scoop gelato into bowls. Top with ground cherries, blondie pieces, walnuts, candied fennel, and fennel fronds.

Do Ahead: Walnuts can be fried 2 days ahead. Let cool; store airtight at room temperature. Fennel can be candied 2 days ahead. Let cool in syrup; transfer to an airtight container and chill.