Melt butter in a large skillet over medium-high heat. Add onions, mushrooms and rosemary; saute 5 minutes. Cool slightly.
In a medium bowl, combine flour, baking powder and salt. With a pastry blender or two knives, cut in shortening until mixture resembles coarse crumbs. Stir in milk and parsley. With floured hands, pat dough into prepared pan, spread onion mixture on top.
In bowl, blend sour cream and egg yolk; spread over onion mixture. Bake for 25 minutes, until golden. Cut into 2-inch triangles.
Makes 40 triangles, about 12 servings.
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