Spicy Shrimp with Creamy Polenta

In a medium, heavy-bottomed saucepan, bring salt and 4 cups water to a boil. Add polenta in a slow steady stream, whisking constantly. Continue to whisk 2 minutes more, then reduce heat to low and simmer, covered, 20 minutes, whisking occasionally. Remove from heat and whisk in butter and cheese. Season with salt. In a large skillet, heat oil over medium-high heat. Add garlic, red pepper flakes, and scallion whites and cook, stirring occasionally, until fragrant, 2 minutes. Add tomatoes and cook until soft, stirring, 4 minutes. Add shrimp and thyme and cook until shrimp are completely pink, stirring, 4 minutes, and season with salt and pepper. Add wine and chicken stock and bring to a boil, then reduce heat to low and simmer until liquid is reduced by half, 2 to 3 minute. Stir in cream and simmer 2 minutes more. Serve shrimp over polenta and garnish with remaining scallion greens.