Preheat oven to 350 degrees. Fill a bowl with ice water; set aside. Spread pecans on a rimmed baking sheet. Bake until fragrant and crisp, 8 to 10 minutes; set aside. (Store up to two days in an airtight container.)
Place 1/2 inch water in a large skillet with a lid. Bring to a boil; salt generously. Add green beans, cover skillet, and cook over medium heat, tossing occasionally, until crisp-tender, 6 to 8 minutes. Transfer immediately to ice water. Drain; pat dry with paper towels. (Cover and refrigerate up to 10 hours.)
In a small bowl, whisk together lemon juice and mustard; season with salt and pepper. Gradually whisk in oil. (Cover and refrigerate up to 10 hours.) To serve, combine all ingredients in a large bowl, and toss with dressing; garnish with fennel fronds, if desired.