Prepare dressing by combining ingredients in a blender or food processor. Blend until smooth, scraping the sides with a spatula to ensure the tahini is thoroughly incorporated. Taste and adjust seasonings, as desired. Set aside.
Preheat oven to 425°F degrees and line a large, rimmed baking sheet with parchment paper or a Silpat® baking mat. Set aside.
Remove the stems and bottom from the fennel bulb, then cut into ½” thick slices from top to bottom. Place slices in a large bowl and reserve the fronds for garnish, if desired. Set aside.
Remove ends and outer skin of red onion. Cut in half from root to stem and then cut each half into ½” thick slices. Place half-round slices into bowl with fennel.
Add olive oil and balsamic vinegar to bowl and season with salt and black pepper, to taste. Gently toss until the fennel and red onion is evenly coated.
Transfer vegetables to prepared baking sheet and arrange in a single layer without overcrowding. Sprinkle with fresh thyme leaves and place in pre-heated oven. Roast for 30-35 minutes, turning once. Once vegetables are golden brown, remove from oven and set aside.
Place arugula and spinach in a large bowl and add 3 tablespoons of lemon-tahini dressing and toss to combine. Arrange greens on serving platter and top with roasted fennel, red onion, and fresh fig slices. Garnish with fennel fronds, if desired.
To serve, drizzle additional dressing on top, if desired, and pass the remaining amount for those who prefer more. Enjoy!