Simmer the lentils: Combine lentils, water, bay leaf, and whole garlic in a saucepan. Bring to a boil, then reduce to a gentle simmer, cover, and cook 20–25 minutes until tender but still firm.
Make the vinaigrette: In a jar, combine olive oil, lemon juice, Dijon, sumac, minced garlic, pepper, and salt. Seal and shake vigorously until emulsified.
Toast the nuts: In a dry skillet over medium heat, toast pistachios 3–5 minutes, tossing, until fragrant and lightly golden. Transfer to cool, then roughly chop.
Warm toss: Drain lentils well; discard bay leaf and garlic. Transfer hot lentils to a bowl, add half the vinaigrette, and toss gently. Let stand 5–10 minutes to absorb and cool slightly.
Final assembly: Add cucumber, bell pepper, red onion, chopped pistachios, apricots, parsley, and mint. Add remaining vinaigrette and toss until coated. Rest 15 minutes before serving; adjust seasoning to taste.