Preheat the oven to 450° F. Toss the onion wedges in 1 tablespoon of oil. Sprinkle with salt and pepper and roast until charred at the edges. (check on them every 5 minutes until done.)
PREPARE THE CHICKPEAS:
Heat remaining 3 tablespoons of oil in a nonstick skillet on medium and add the mustard seeds. When they begin to pop, cover the pan and let them finish (20 to 30 seconds). Immediately turn down the heat to prevent the seeds from burning and add garlic and cumin seeds. Toss until the garlic and cumin are toasted. Add the chickpeas and toss to coat in the spices and oil. Remove from heat.
ASSEMBLE THE SALAD:
Place the greens in each of four bowls. Divide the fresh basil between the bowls. Sprinkle with the chickpeas. Divide the roasted onions between the salads. Squeeze lemon juice over each salad and sprinkle with kosher salt. Serve immediately.