Bring a large pot of salted water to a boil. Add orzo pasta and cook until al dente, about 8-10 minutes. Reserve 1/2 cup pasta water, then drain orzo.

In a large skillet, melt butter over medium-high heat. Season shrimp with salt and pepper, then sear in the skillet for 2-3 minutes on each side until pink and opaque. Remove and keep warm.

In the same skillet, lower heat to medium and add remaining butter. Add minced garlic and sauté for 30 seconds until fragrant. Add spinach and stir until wilted, about 2-3 minutes.

Return shrimp to skillet and mix in reserved orzo, lemon zest, and juice. Add reserved pasta water gradually, mixing for 2 minutes.

Taste and season with additional salt, pepper, and red pepper flakes. Heat through for another minute.

Serve immediately, garnished with additional lemon zest and red pepper flakes if desired.