"Seder Plate" Salad

Preheat oven to 350 °F. Toast walnuts on a rimmed baking sheet, tossing once, until golden brown, 8 –10 minutes. Let cool; coarsely chop. Heat 1 Tbsp. oil in a medium skillet over medium-high. Cook sausage, turning occasionally, until browned on both sides, 3 –5 minutes. Transfer to a plate. Meanwhile, whisk horseradish, lemon juice, honey, 1/4 tsp. salt, and 1/8 tsp. pepper in a medium bowl. Slowly whisk in remaining 3 Tbsp. oil in a steady stream. Coarsely chop 2 Tbsp. parsley and stir into dressing. Toss romaine, apple, sausage, and remaining parsley in a large serving bowl. Add two-thirds of the dressing and toss to coat; season with salt and pepper if necessary. Top with walnuts and eggs; drizzle with remaining dressing.