Rinse 1 cup lentils under cold water and drain.
Bring 4 cups of water to a boil in a medium pot, add the lentils, reduce heat to medium-low, cover, and simmer until firm-tender, about 20 minutes.
While lentils cook, whisk together 2 Tbsp balsamic vinegar, 1 tsp Dijon mustard, 1 clove minced garlic, and 2 Tbsp capers in a small bowl to make the dressing.
Roughly chop the spinach and chives, and dice the tomatoes; place them in a large bowl.
When lentils are cooked, drain them and immediately add the hot lentils to the bowl with the vegetables so the spinach wilts slightly.
Add the dressing to the lentils and vegetables, toss to combine, and adjust seasoning with salt and pepper to taste.