Prepare the dressing by combining all ingredients in a blender or food processor. Blend until smooth, and scrape the sides with a spatula, as necessary, to ensure that the tahini is thoroughly incorporated. Taste and adjust seasonings to your preference. Set aside.
Preheat the oven to 425°F degrees and line a large baking sheet with parchment paper. Set aside.
Remove stems and bottom from the fennel bulb, then cut into ½" thick slices from top to bottom. Place slices into a large bowl and reserve the fronds for garnish. Set aside.
Remove the ends and outer skin of the red onion. Cut in half and then cut into ½ inch thick slices. Place into a bowl with fennel.
Add the olive oil and balsamic vinegar to the bowl and season with Himalayan salt and black pepper to taste. Gently toss everything together until the fennel and red onion are coated evenly.
Transfer vegetables to the prepared baking sheet arranging in a single layer to avoid overcrowding. Sprinkle with fresh thyme leaves, then place in a pre-heated oven. Roast for 30-35 minutes, turning halfway through. When the vegetables are golden brown, remove them from the oven and set them aside.
Place the arugula and spinach in a large bowl, then add 3 tablespoons of the lemon-tahini dressing. Toss to combine. Arrange greens on a serving platter, then top with the roasted fennel, red onion, and sliced fresh fig. Garnish with fennel fronds.
To serve, drizzle extra dressing on top if desired. Enjoy!