Place the cauliflower in a large pot and fill with water. Place on the stove on high heat and bring to a boil. This will take some time with a large pot. Once at a boil, allow it to boil for 15 minutes. Continue with the cauliflower on step 4.
While the cauliflower is boiling, peel the shrimp and butterfly them (making a cut in the back and extracting the vein). In a bowl, toss the shrimp with salt and pepper.
Heat 4 Tablespoons of butter in pan on medium to high heat. Add shrimp, scallions, and garlic and cook each side for 2-3 minutes. Squeeze lemons on shrimp about halfway through cooking.
Once the cauliflower is done boiling, drain the water but leave the cauliflower in the pot. Place the pot of cauliflower back on the stove and turn the heat on low. Add 1 Tablespoon of butter, and salt and pepper to taste. Mash it with either a fork or a potato masher.
Assemble the dish by placing a scoop of cauliflower mash and then topping with the shrimp.