Preheat the oven to 425°F (220°C) while preparing vegetables. Toss beet and sweet potato cubes with olive oil, salt, and pepper until evenly coated. Spread them in a single layer and roast until tender and lightly caramelized.
While vegetables roast, blend ricotta with lemon juice, olive oil, and salt until smooth. Blend longer for a fluffier texture if desired. Taste and adjust seasoning before setting aside.
Whisk tahini with lemon juice and sweetener in a bowl. Slowly add warm water while whisking until the dressing becomes pourable. Add cumin if desired and adjust seasoning.
Place fresh mixed greens in a serving bowl or platter. Add warm roasted vegetables evenly over the greens. Arrange sliced avocado on top for creamy texture.
Spoon dollops of whipped ricotta across the salad. Drizzle generously with lemon-tahini sauce and finish with herbs or crunchy toppings before serving immediately.