Preheat oven to 350 degrees. Spread pecans on a rimmed baking sheet and toast until fragrant and lightly browned, about 10 minutes; set aside.
Meanwhile, with the shredding disk of a food processor, shred Brussels sprouts (if you don't have a food processor, halve and thinly slice with a chef's knife). In a large nonstick skillet, heat butter and oil over medium-high heat. Add mustard seeds and cook, stirring occasionally, until fragrant, about 30 seconds. Add Brussels sprouts and cook, tossing occasionally until tender and beginning to brown, 7 to 9 minutes. Remove from heat, stir in lemon juice, and season with salt and pepper. Serve topped with pecans.