Suprȇme 1 orange and set aside. Zest the remaining orange to get ½ teaspoon zest, then juice it to get 2 tablespoons juice; combine the zest and juice in a large bowl. Whisk in 1½ tablespoons lemon juice, chopped shallot, 1½ teaspoons mustard and ¼ teaspoon salt; let stand for at least 10 minutes.
Meanwhile, place ¾ cup walnuts in a small skillet. Cook over medium heat, stirring often, until fragrant and brown, 3 to 5 minutes.
While whisking the lemon juice mixture constantly, slowly stream in 3 tablespoons oil until combined. Add chopped avocado, 1 (6-ounce) package raspberries, 1 (5-ounce) package spinach and the orange segments; gently toss until evenly coated. Sprinkle with the toasted walnuts.
