Preheat oven to 350°F. Cook shells until al dente, rinse, set aside.
In small frying pan, melt 1 tablespoon butter over medium heat and add rosemary. Cook for about 4 minutes or until fragrant and leaves are tender. Add pears and raise heat to medium-high, sauté for a few minutes and add 1 teaspoon sugar. Let cook until pears are golden brown and slightly softened; about 10 minutes. Set mixture aside.
In a large saucepan, melt 1 tablespoon butter over medium heat. Add chopped onion, 1 teaspoon salt and 1 teaspoon sugar, and cook until translucent and slightly caramelized. Add 1 tablespoon butter and when melted, add flour; whisking constantly to avoid lumps. When roux is light brown and bubbly, whisk in cream, stirring constantly until sauce starts to thicken. Add gorgonzola, a little at a time, and stir until melted. Add cooked pasta and pears and stir until evenly distributed. Salt and pepper to taste. (It should look a little soupy)
In buttered cast iron skillet, add pasta mixture and smooth over – so the surface is even. Add crumbled crackers over entire surface, and dot with remaining 2 tablespoons butter. Bake until top is crispy and golden, about 30 minutes. Let rest 5 minutes and serve.