Preheat oven to 375°F. Cook pasta according to package directions until al dente; drain and set aside.
Meanwhile, wash mushrooms and use knife to carefully remove gills. Slice mushroom caps into mid-size chunks; set aside.
Add 2 tablespoons butter to large sauté pan over medium heat. Add sliced onions and cook, stirring often, 10-12 minutes, until onions softens and becomes golden brown. Add mushrooms and season with 1 teaspoon salt, 1 teaspoon pepper and garlic salt. Cook until mushrooms begin to soften, approximately 3-5 minutes. Remove from heat, set aside.
In small sauce pan, combine milk and half and half. Warm over low heat. In large sauce pan, melt remaining butter and gradually whisk in flour until mixture thickens. Do NOT let burn. Slowly whisk in warm milk mixture, working in small amounts and incorporating each pour in to roux. Slowly add shredded cheese in small amounts, whisking after each addition until cheese is melted and well incorporated.
Whisk in mustard and remaining 1 teaspoon each salt and pepper. Let sauce thicken slightly over low heat for 3-4 minutes.
Divide pasta among six ovenproof ramekins and top with onion-mushroom mixture. Pour cheese mixture over mushrooms; top with rosemary and breadcrumbs. Bake 25-30 minutes or until breadcrumbs are golden brown.
If you can’t find BellaVitano Cheese, substitute Asiago.