Heat oven to 350°F/180°C.
Toss the sweet potatoes with olive oil and salt, then spread on a parchment-lined sheet pan with the leek. Roast until tender and lightly caramelized, about 30 minutes.
Arrange the romaine, arugula, lentils, roasted vegetables, and olives on a serving platter.
Whisk the tahini, mustard, honey, garlic, salt, lemon juice, olive oil, and water until smooth and pourable. Adjust with more water as needed.
Drizzle the dressing over the salad, then finish with feta and pumpkin seeds. Serve immediately.