Rinse the lentils under cold water using a fine-mesh strainer and pick out any debris.

Add lentils, 480 ml water, and 2 g salt to a small saucepan, bring to a boil, then reduce heat and simmer for 18–22 minutes until tender but not mushy.

Drain lentils if needed and spread them on a plate to cool slightly.

Rinse the quinoa thoroughly under cold water in a fine-mesh strainer to remove bitterness.

Add quinoa, 480 ml water, and 2 g salt to a medium saucepan, bring to a boil, then reduce heat to low, cover, and cook for 15 minutes until water is absorbed.

Remove quinoa from the heat and let it sit covered for 5 minutes, then fluff with a fork and let cool slightly.

While the grains cool, chop cucumber, cherry tomatoes, red onion, parsley, and mint, and set aside.

In a small bowl, whisk together olive oil, lemon juice, red wine vinegar, Dijon mustard, minced garlic, remaining 1 g salt, and black pepper until emulsified.

Add the warm quinoa and lentils to a large mixing bowl and drizzle over about two-thirds of the dressing, tossing gently to coat.

Let the mixture rest for 10 minutes to absorb the flavors.

Add cucumber, cherry tomatoes, red onion, parsley, and mint to the bowl and toss to combine.

Taste and add more dressing, salt, or pepper as needed.

Gently fold in crumbled feta cheese if using, just before serving.

Serve the salad slightly warm or at room temperature, or chill for 30–60 minutes for a colder salad.