Preheat oven to 375°F.
On one sheet pan, place the peeled and diced beets in the center. Drizzle with about a tablespoon of olive oil and season with a good pinch of Kosher salt. Spread out evenly on the sheet pan and place in the oven. Baking time will vary, 30-45 minutes. Roast, stirring occasionally, until fork-tender.
On a second sheet pan, place the peeled and diced sweet potatoes. Drizzle with about a tablespoon of olive oil and season with a good pinch of Kosher salt. Spread out evenly on the sheet pan and place in the oven. Baking time will vary, 20-30 minutes. Roast, stirring occasionally, until fork-tender.
While the beets and sweet potatoes are roasting, begin the lentils.
Rinse and drain the lentils. Add them to a saucepan with the fresh rosemary, thyme, smashed garlic, and onion piece. Season with about ½ teaspoon of Kosher salt, then add enough water to the pot to cover the lentils about 2 inches. Press the herbs, onion, and garlic into the lentils. We used about 3 ½ cups of water. Cover saucepan.
Bring to a boil over high heat; reduce the heat and simmer, covered, until the lentils are tender, about 25 minutes. Strain the lentils and discard the garlic, onion, and herb sprigs. There will be individual pieces of Rosemary in the lentils, this is okay.
While the lentils are simmering, prepare the salad dressing: In a jar, add all the ingredients and shake well.
For the assembly, place the strained, warm lentils in a bowl, the roasted sweet potatoes, and top with the roasted beets. Pour about 1/3 of the dressing over the mixture and gently toss.
Cool the mixture thoroughly and refrigerate until ready to serve.
Remove the salad from the refrigerator 30 minutes before serving. Shake the remaining dressing, then dress the salad to taste. Season with additional salt and pepper if necessary.