Preheat the oven to 400 degrees.
In a medium saucepan, bring the lentils, bay leaf, 6 cups of water, and 2 teaspoons salt to boil. Simmer the lentils until tender, about 20 to 25 minutes. Drain and remove the bay leaf.
Meanwhile, on a parchment lined baking sheet, toss the potatoes with 1 tablespoon of olive oil and ½ teaspoon salt. Arrange in an even layer, cut-side down and roast in the oven until tender and lightly browned, 25 minutes.
In a large salad bowl, combine the shallot, garlic and vinegar. Allow to sit for 5 minutes to mellow. Whisk in the mustard, followed by 1 teaspoon of olive oil. When the oil has incorporated, add another splash of oil. Repeat this process until the vinaigrette is thick and smooth, with just a slight bite. You’ll probably use about ¼ cup of olive oil total. Season to taste with salt.
Add the lentils, potatoes, and herbs to the mixing bowl and toss to combine. Serve at room temperature or chill until ready to serve.