Place all ingredients in a bowl and whisk until well mixed. Cover and refrigerate until service.
In a bowl, whisk together buttermilk and hot sauce. Add mushrooms and toss to ensure they are evenly coated. Cover and refrigerate for 1 hour.
In a bowl, combine flour and OLD BAY® Seasoning. Remove mushrooms from buttermilk. Dredge a small amount of mushrooms at a time in flour mixture, gently pressing flour into the mushrooms to create ridges. Lay dredged mushrooms out on a rack. Continue until all mushrooms are coated in flour.
Sift flour mixture into a new bowl and discard clumped bits. Dredge mushrooms a second time and lay out on a rack. Cover and refrigerate until service.
Set fryer to 350°F. For each serving, fry 6-8 breaded mushrooms depending on size. Fry to golden brown. Drain.
Spread 2 tablespoons of OLD BAY Hot Sauce Tiger Sauce on the insides of the baguette. Place 2 halves of lettuce and 2 slices of tomato on bottom of the baguette. Top with fried mushrooms. Serve warm.
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