Mince garlic (or shallot). Rinse mushrooms and trim stems, then finely chop.
Melt butter in a large skillet over medium-low heat. Add garlic and mushrooms. Cook and stir until soft and most of the moisture has cooked off, about 5 to 7 minutes.
Remove from heat and stir in cheese, sour cream (or yogurt), mustard, rosemary, salt, pepper, and red pepper flakes. NOTE: You can make ahead to this point, and refrigerate until ready to assemble and cook.
To assemble and cook:
Preheat broiler about 5 to 10 minutes before ready to cook.
Slice half of the baguette into 1/2-inch-thick pieces, cutting straight across for smaller rounds, or on the diagonal for larger, oblong shapes.
Place slices of bread on an ungreased baking sheet. Spread about 1 tablespoon of the mushroom mixture on each. Cut more of the baguette as needed to use up the mushroom spread.
Top some or all pieces with a small slice of cheese, if desired.
Broil 3 to 4 minutes, or until bubbling and lightly browned.
Garnish with fresh parsley, either by chopping and sprinkling some over the melts, or by placing sprigs around them.