In a medium saucepan, combine 1 cup of rinsed lentils with 3 cups of water or vegetable broth. Bring to a simmer over medium-high heat, then reduce to medium-low, cover, and cook until tender, about 20-25 minutes. Once done, drain and rinse under cold water to cool.
While the lentils cook, whisk together 3 tablespoons of olive oil, 2 tablespoons of lemon juice, 1 teaspoon of Dijon mustard, minced garlic, sea salt, and black pepper in a small bowl until well combined.
In a large bowl, add the cooled lentils, diced cucumber, red onion, chopped mint, and sun-dried tomatoes. Pour the dressing over the salad and toss to ensure everything is evenly coated.
Serve and enjoy.