In a small skillet, heat oil over medium heat; add cumin and garlic. Cook and stir for 1 minute until toasted and fragrant; set cumin oil aside to cool completely.
Stir together cucumber, 1/4 cup walnuts, lemon juice, yogurt, salt, and pepper.
Stir in cumin oil and top with additional 2 tablespoons walnuts. Serve immediately.