Place walnuts in a small skillet over medium heat; cook 3 minutes or until walnuts are lightly browned, shaking pan frequently. Remove from heat.
Steam Brussels sprouts, covered, 10 minutes or until tender. Add cauliflower to pan; steam, covered, 2 minutes or just until tender. Drain. Combine Brussels sprouts, cauliflower, salt, pepper, and juice in a medium bowl; toss to combine. Sprinkle evenly with walnuts.