Bring a large saucepan of water to a boil. Add the rice and cook, stirring a few times, until just al dente, 16 minutes. Drain well.

Meanwhile, in a medium saucepan, heat the oil. Add the shrimp shells; cook over moderately high heat, stirring often, until beginning to brown, about 3 minutes. Add the chopped fennel, onion, celery, carrot, garlic, fennel seeds, and Aleppo pepper and cook over moderately high heat, stirring often, until the onion is golden, about 6 minutes. Add the wine and boil until almost evaporated. Add 2 cups of the chicken stock and simmer over moderate heat for 12 minutes. Strain.

In another medium saucepan, combine the cream with the saffron, 1/2 cup of the shrimp stock, and the remaining 1 cup of chicken stock and bring to a simmer. Add the parboiled rice and cook over moderate heat, stirring, until the sauce has thickened and the grains are tender, about 3 minutes. Season with salt and black pepper, cover, and remove from the heat.

In a large skillet, bring the remaining shrimp stock to a simmer. Add the shrimp and cook over moderately low heat, turning once, until pink and curled, about 3 minutes. Remove the skillet from the heat and stir in 4 tablespoons of the butter, 1 tablespoon at a time. Season with salt and black pepper.

In a small bowl, toss the sliced fennel and scallion with the lemon juice and season with salt and black pepper.

Rewarm the rice and stir in the remaining 1 tablespoon of butter. Transfer the rice to shallow bowls. Spoon the shrimp and sauce over the rice. Top with the fennel salad, garnish with the thyme, and serve.

The shrimp stock can be refrigerated overnight.