Preheat the oven to 425 degrees . In a medium bowl, stir the marinara, cumin and cinnamon; season with salt and pepper. Reserve 3 tbsp. chopped onion for the salad. Add the remaining onion, the lamb and garlic to the marinara mixture and stir until well combined.
Place the naan on a large baking sheet. Spread each naan with some of the lamb mixture, leaving 1/2-inch borders; top with the feta. Bake until the naan is crispy and the lamb is cooked through, about 18 minutes. Turn on the broiler and broil until the naan is crispy and brown, about 2 minutes.
Meanwhile, in a large bowl, whisk the oil and vinegar; season. Add the watercress and the reserved chopped onion and toss to coat. Serve the salad on top of the pizzas.