Warm 1 tablespoon oil in a large pot over medium heat. Add half the sliced garlic and cook until it just starts to turn golden, 1–2 minutes. Add the spinach and 1/4 teaspoon salt; toss and cook until mostly wilted, 3–5 minutes. Off the heat, stir in the lemon juice. Transfer the spinach to a bowl and keep warm.
Return the pot to medium-high and add the remaining 2 tablespoons oil. Add the remaining garlic; cook until lightly golden, 1–2 minutes. Add the shrimp, crushed red pepper and remaining 1/8 teaspoon salt; cook, stirring, until shrimp are opaque and just cooked through, 3–5 minutes. Spoon the shrimp over the spinach and finish with parsley and lemon zest.