Make the shrimp broth. Add the reserved shrimp shells and tails to a medium saucepan along with 3 cloves of garlic, 1 teaspoon salt, 1/2 teaspoon pepper, and 7 cups of water. Bring everything to a boil over medium-high heat. Reduce the heat to low and simmer for about 20 minutes.
Strain the broth. Strain the shrimp broth into a large measuring cup or bowl and discard the shells and garlic. You should have approximately 6 cups of broth. If you are a little short, add enough water to make 6 cups. Return the broth to the saucepan and keep it warm over low heat.
Cook the shrimp. Heat a large skillet or braiser over medium-high heat. Add 1 tablespoon of olive oil. Once the oil is hot, add the chopped shrimp and season generously with salt and pepper. Cook, stirring occasionally, until the shrimp are pink and cooked through, about 5 minutes. Transfer them to a plate and set aside.
Start the risotto. Thinly slice the remaining 3 cloves of garlic. Reduce the skillet to medium-low heat and add another tablespoon of olive oil if the pan seems dry. Add the garlic and cook for about 1 minute, stirring frequently.Add the Arborio rice and stir until the rice is coated in the oil.Pour in the lemon juice and cook, stirring occasionally, until most of the liquid has been absorbed, about 2 minutes. Slowly add the broth. Add about 1 cup of the warm shrimp broth to the rice. Cook while stirring gently and continuously until most of the liquid has been absorbed, about 2 to 3 minutes.Continue adding the broth about one ladleful at a time, allowing the rice to absorb the liquid before adding more. Cook until creamy. Continue this process for about 20 to 25 minutes, until the rice is creamy and al dente. If you run out of broth before the rice is completely cooked, add a little warm water.
Add the spinach, shrimp, and green onions. Stir the cooked shrimp back into the risotto along with the chopped spinach, sliced green onions, and about half of the lemon zest. Cook just until the spinach begins to wilt and everything is warmed through.
Garnish and serve. Spoon the risotto into bowls and finish with the remaining lemon zest, fresh spinach, and sliced green onions.Serve immediately while it is warm and creamy.