How to Make It
Preheat the oven to 450°. On a rimmed baking sheet, toss the cauliflower with 2 tablespoons of the olive oil and season with salt and pepper; spread in an even layer. On a second rimmed baking sheet, toss the eggplant with 1/4 cup of the olive oil and season with salt and pepper; spread in an even layer. Roast the vegetables, stirring occasionally, until golden and tender, 20 to 25 minutes. Transfer the vegetables to racks to cool.
Meanwhile, in a food processor, puree the yogurt, tahini, anchovies, garlic and lemon juice until smooth. With the machine on, drizzle in 3 tablespoons of water and the remaining 3 tablespoons of olive oil until smooth; season with salt and pepper. Stir in the dill and parsley.
In a 3- to 4-quart glass trifle dish or bowl, spread the cucumbers in an even layer; season with salt and pepper. Top with the cooled eggplant and cauliflower and the avocados and season with salt and pepper. Finish with the watercress and serve the salad with the tahini dressing.