Preheat oven to 375ºF.
Place a large pot of water over high heat to boil the pasta. When the water reaches a bubble, salt it well, drop the shells in and cook them to al dente according to package directions. Drain the cooked shells and shock them under cold water to stop them from cooking. Reserve.
While the shells are cooking, mash up the tomatoes in the bottom of a casserole dish. Stir in the butter, basil, some salt and pepper into the dish, and set aside.
Place a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the garlic to the pan and cook until aromatic and light golden brown, about 1 minute. Add the spinach, and nutmeg to the pan, and cook to heat through.