Pulse about half of the cauliflower in a food processor until the florets break down into finer pieces about the size of grains of rice (it's totally fine if they clump; think of this step as cauliflower "rice" gone wrong). Transfer to a medium saucepan, pulse the remaining cauliflower and add that to the pan too. Add the milk, 1 tablespoon of the butter, 1/2 teaspoon salt and several grinds of pepper and bring to a simmer over medium-high heat. Simmer, stirring frequently, until the mixture is soft and smooth and looks like grits, about 10 minutes. Remove from the heat, stir in the Parmesan and adjust the seasoning with more salt and pepper if you'd like. Cover and keep warm.
Season the shrimp with salt and pepper. Melt the remaining 2 tablespoons butter in a large skillet over medium-high heat. Add the shrimp, garlic and cayenne if using and cook, tossing, until the shrimp are pink and just cooked through, 3 to 4 minutes. Remove from the heat, add the parsley, lemon juice and 1 tablespoon water and stir to coat the shrimp with the sauce. Season with salt and pepper.
Divide the cauliflower grits among shallow bowls and top with the shrimp and sauce. Serve with lemon wedges.
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