Preheat the oven to 425°F. Add the French lentils, 2 cups water, and a pinch of salt to a small saucepan over high heat. Bring the lentils to a boil, reduce heat to low, and simmer until tender, about 18 to 20 minutes. Drain any remaining water and season with salt and pepper.
Add 1 inch salted water to a large skillet, bring to a boil for the green beans in in the next step. Quarter the potatoes. Thinly slice the garlic. Trim the green beans. Thinly slice the cucumber into rounds. Transfer the potatoes to a baking sheet and toss with the French mustard herb blend, 1 tsp olive oil, and a pinch of salt and pepper. Roast until the potatoes are browned and tender, about 18 to 22 minutes.
Once the large skillet of water is boiling, add the green beans and cook until bright green and just tender, about 1 minute. Drain the beans and rinse with cold water to stop the cooking process.
Dry the skillet and return to medium heat with 2 tbsp olive oil. Add the sliced garlic and cook until fragrant, about 30 seconds. Add the cherry tomatoes, sprinkle with salt, and cook until they soften and begin to blister, about 4 to 6 minutes.
Transfer the garlic and tomatoes to a cutting board and roughly chop. In a medium bowl, combine the garlic, tomatoes, 1 tbsp olive oil, lemon juice, ¼ tsp salt, and a pinch of pepper. Pick the tarragon leaves, add to the bowl, and toss to combine. Taste the vinaigrette and adjust seasoning with salt if necessary.
Divide the spinach between plates or large bowls. Add the French lentils, roasted mustard potatoes, green beans, and sliced cucumber. Drizzle the Nicoise salads with cherry tomato vinaigrette and sprinkle with walnuts and kalamata olives.