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Heat 1 T margarine & 1 T oil in large skillet over medium-low flame.
Add onions and cook 18 minutes, stirring often, until limp and deeply colored. Transfer onions to small bowl.
Wipe out skillet, add remaining butter and olive oil and heat over medium-high.
Cook mushrooms in oil mixture 5 minutes. Strin in rosemanry, salt, & pepper, then cook 5 minutes more until mushrooms are tender.
Stir in mustard and half & half; cook, stirring often, until thickened into sauce. Add onions.
Season meat with more salt and pepper.
Form into six large, 1-inch-thick patties. Heat large, cast iron skillet over medium-high flame for several minutes.
Pan-grill to desired doneness, 4-6 minutes per side.
Split buns in half and lightly toast. Serve burgers, topped with mushroom sauce, on toasted buns with dijon mustard.