Cook the pasta in a large pot of well-salted boiling water over high heat until al dente, about 9 minutes.
Meanwhile, heat the broiler to medium. Place the boursin and cream cheese in a small saucepan over medium heat; cook, mixing well, until the cheese melt and form a sauce (or microwave in a bowl, 1 1/2 minutes). Drain the pasta, reserving 1/2 c pasta water. Return the pasta to the pot, fold in the melted cheese mixture, pasta water and the mozzarella until well combined. Stir in the spinach just until incorporated.
Pour mixture into a buttered baking dish, 8 to 9 inches and 3 inches deep. Sprinkle with the breadcrumbs and Parmesan; broil until golden brown, about 3 minutes.