Preheat the broiler. Bring a large pot of salted water to a boil. Melt 2 tablespoons butter in a large skillet over medium-high heat. Add the onion, mushrooms, 1/4 teaspoon salt and a few grinds of pepper. Cook until the vegetables are beginning to brown, 5 to 8 minutes. Add the sausage and cook, breaking up the meat, until no longer pink, about 4 minutes. Add the sliced garlic and marjoram and cook until softened, 1 more minute. Add the tomatoes and 1/4 cup water, bring to a simmer and cook until the vegetables are tender and the sauce has thickened slightly, about 6 minutes.
Meanwhile, cook the ravioli in the boiling water as the label directs. Reserve 1/4 cup cooking water, then drain. Add the ravioli and the reserved cooking water to the sauce; stir to coat. Transfer half of the ravioli and sauce to a 9-by-13-inch broiler-proof baking dish. Sprinkle with 1/2 cup mozzarella. Top with the remaining ravioli, sauce and mozzarella.
Meanwhile, microwave the remaining 1 tablespoon butter in a small microwave-safe bowl until melted. Stir in the grated garlic and breadcrumbs; season with salt and pepper. Sprinkle over the baked ravioli and broil until browned on top, about 2 minutes. Top with the parsley.