Use a sharp chef's knife and a mandoline to thinly slice the trimmed Brussels sprouts into fine strips. Place the slices into a large mixing bowl.
Heat a dry skillet over medium heat and add the mixed seeds. Toast, shaking the pan frequently, until the seeds are golden and fragrant, about 5-7 minutes. Remove from heat and set aside.
Add 2 tablespoons of olive oil to the skillet used for toasting seeds and heat over medium until shimmering. Add the toasted seeds back to the pan and toss to combine, allowing their flavors to meld for 1 minute. Season with salt and black pepper to taste, then transfer mixture to the mixing bowl with the raw Brussels sprouts.
Gently toss the mixture in the bowl until the shredded Brussels sprouts are evenly coated with the toasted seeds and oil. Serve immediately for a crisp, fresh salad texture.