Rinse lentils under cold water until clear. In a pot, boil 4 cups of salted water, add lentils, reduce heat, and cook for about 20 minutes until tender. Drain and cool.

In a large mixing bowl, combine cooled lentils, spinach, cherry tomatoes, and red onions. Toss gently to distribute.

In a small bowl, whisk together olive oil and citrus juice until emulsified and glossy.

Drizzle dressing over the lentil and vegetable mixture and toss gently until well-coated. Season with salt and pepper to taste.

Serve immediately or chill for 15 minutes. Best served cold or at room temperature.