Preheat your oven to 400F and line a baking sheet with parchment. Toss the cubed sweet potato in oil and place them on the baking sheet. Transfer the sheet to the oven. Roast the sweet potatoes for 35-40 minutes, stirring once halfway through, or until the sweet potatoes are tender and crispy at the edges.
While the potatoes roast, bring a pot of water to boil. Add the lentils and cook for 20-30 minutes, or until the lentils are tender but not mushy. Drain the lentils. (If you're using pre-cooked/canned lentils, you can skip this step.)
Whisk together the tahini, water, vinegar, mustard, maple syrup, and salt.
Place the roasted sweet potatoes, cooked lentils, tomatoes, and arugula into a large mixing bowl. Add the dressing. Toss well to combine. Taste the salad and add a pinch of salt if desired and black pepper to taste. Serve the recipe on its own or with whole grain toast or crackers.