Make dressing: Whisk together the Dijon, vinegar, olive oil, salt, and pepper until emulsified. Set aside.
Preheat the oven to 275°. Quarter tomatoes and place skin-side down (or else they’ll stick) on a baking sheet lined with parchment paper or foil. Drizzle over the olive oil and balsamic vinegar and sprinkle with some salt. Toss the tomatoes gently using your hands to make sure all of them are coated in a little oil. Roast for 1½ hours.
Bring lentils and 6 cups of water (or broth) to a boil and simmer until tender but still holding their shape, 15-20 minutes. Drain and let cool.
Gently toss lentils with tomatoes, feta, dill, chives, mint, red onion, and dressing. Season with salt and pepper to taste.
