Preheat oven to 425°F. Cut 1 head cauliflower into florets and spread in one layer on a large rimmed baking sheet. Drizzle with 2 Tbsp. of olive oil and season with salt and pepper. Roast for 20-25 minutes, or until cauliflower is tender and golden brown.
Transfer cauliflower to food processor and add butter: Process, scraping sides of processor as needed, until cauliflower has the texture of smooth grits or polenta. Add about ½ of the jar of sauce and pulse to combine. Add additional salt and pepper to taste. Set aside.
Next, make the polenta. Bring water to a boil in a medium saucepan over medium high heat. Add butter. While whisking gently, pour the polenta into the boiling water in a steady stream. Turn down heat to low and continue whisking until the polenta has thickened enough that it doesn't settle on the bottom of the pan. Cook polenta for 5-7 minutes over medium heat, stirring occasionally, making sure polenta doesn't get too thick. Keep adding small amounts of water if polenta gets too thick.
Add the remaining sauce and the prepared cauliflower to the cooked polenta and stir well. Cover and keep warm.
Next, roast the shrimp and tomatoes. Spread the shrimp on a rimmed baking sheet. Add the tomatoes and the garlic to the baking sheet and drizzle 2 Tbsp. of olive oil over all. Season with salt and pepper and toss so that the shrimp and tomatoes are coated with the oil and garlic. Roast at 425°F for 7-8 minutes, or until shrimp is cooked through and tomatoes are bursting and starting to char. Remove the shrimp and tomatoes from the oven and add the arugula to the pan. Using tongs, toss the arugula with the hot shrimp and tomatoes so that it wilts slightly.
To serve, spoon some polenta onto a plate or into a shallow bowl. Top with the shrimp and tomato mixture. Sprinkle with pine nuts and serve.