Preheat oven to 425°F. Cut lemon in half crosswise. Thinly slice 1 lemon half; remove and discard seeds and lemon end. Juice the remaining lemon half to yield 1 tablespoon juice; set aside. Place the lemon slices, tomatoes, leeks, garlic, 2 tablespoons oil and 1 tablespoon capers in a 9-by-13-inch baking dish; toss to combine.

Bake, uncovered, until the tomatoes have begun to burst and the lemon has softened, 15 to 18 minutes. Remove from oven; gently stir in broth, orzo and 1 teaspoon salt. Cover with foil. Bake until most of the liquid has been absorbed and the orzo is al dente, 20 to 26 minutes.

Meanwhile, season salmon and asparagus with pepper and the remaining 1 teaspoon salt; drizzle with 2 tablespoons oil.

Once the orzo is al dente, remove the baking dish from the oven; stir in the asparagus and 2 tablespoons dill. Nestle the salmon into the orzo mixture. Bake, uncovered, until the salmon is cooked through and flakes easily when tested with a fork, about 10 minutes.

Meanwhile, combine the reserved 1 tablespoon lemon juice and the remaining 2 tablespoons oil, 1 tablespoon capers and ¼ cup dill in a small bowl. Serve the sauce alongside the casserole.