How to Make It
In a 4-quart saucepan, bring 3 cups of water to a boil. Stream in the polenta, whisking constantly, and bring to a boil. Lower the heat to a simmer and cook for 20 minutes, stirring frequently. Set aside, covered with plastic wrap.
In a very large skillet, heat the olive oil until just smoking. Add the garlic and cayenne chile and cook until softened, about 30 seconds; do not brown the garlic. Add the wine and bring to a boil, then add the shrimp and cook until just pink and curled; if using crab, cook just until warmed through. Remove from the heat. Add the butter, lemon juice and parsley and season with salt and pepper.
Place 1/4 cup of the soft polenta in the center of each plate, spoon the shrimp or crab mixture on top and serve.