Preheat the oven to 375 degrees . Grease a 10-inch springform pan or 4-quart ovenproof bowl. Line the bottom and sides of the pan with bread slices, fitting them tightly; reserve several bread slices for the top.
In a large pot of boiling, salted water, cook the pasta until al dente.
Meanwhile, in a large skillet, melt 2 tablespoons butter over medium heat. Add the onion and cook until softened, about 5 minutes. Stir in the peas and cook for 1 minute. Stir in the tomatoes and heavy cream, mixing until combined, and bring to a simmer. Set aside 1 cup of the sauce.
Drain the pasta, add to the sauce in the skillet and toss to coat. Stir in the mozzarella and half of the parmesan. Season with salt. Pour the pasta into the bread-lined pan, pressing down with a wooden spoon. Cover with the remaining bread slices. Sprinkle the remaining parmesan on top and dot with the remaining 2 tablespoons butter.
Bake the pasta until golden and crusty, about 40 minutes (loosely cover with foil if necessary to prevent overbrowning). Let cool for about 30 minutes, then unmold and slice. Serve with the reserved tomato sauce and extra parmesan.