Step 1

Using a vegetable peeler, remove zest from lemon in 3"-long strips. Shave off any white pith and set zest aside. Cut any remaining white pith from lemon, then cut along sides of membranes to release segments onto a cutting board; remove seeds. Coarsely chop segments; place in a medium bowl. Pulse reserved lemon zest and sugar in a food processor until lemon zest is finely chopped, about 1 minute. Mix into lemon segments.

Step 2

Place cranberries and prunes in food processor (no need to wash out) and pulse until coarsely chopped; add to lemon mixture. Mix in brandy and ¾ tsp. salt. Transfer cranberry mixture to an airtight container, cover, and let sit at room temperature 12 hours. Mix mustard powder, mustard seeds, vinegar, and 1 Tbsp. oil into cranberry mixture.

Step 3

Place a rack in middle of oven; preheat oven to 450°. Toss brussels sprouts and ½ cup oil on a rimmed baking sheet; season with salt and pepper. Roast until browned and tender, 25–30 minutes.

Step 4

Add 1 cup mostarda to brussels sprouts and toss to combine. Transfer to a platter or large bowl; drizzle with oil.

Step 5

Do Ahead: Mostarda can be made up to 1 month ahead. Keep chilled.