To make the salad: Preheat the oven to 375°F. Line a baking sheet with parchment paper.
In a bowl, toss the beets, sweet potatoes, and fennel with the 2 teaspoons balsamic vinegar. Place on the baking sheet and cover with foil. Roast for 30 to 35 minutes, or until the veggies have softened but are still somewhat firm.
To make the dressing: In a small bowl, mix together the 2 tablespoons balsamic vinegar, the water, tahini, dill, salt, garlic, and pepper to taste.
Transfer the cooked vegetables to a serving bowl and toss them with the dressing.