For filling: Blanch the green beans in boiling water 2 minutes; drain well. In a large saucepot melt butter over medium heat. Add onion and sauté until softened. Stir in the flour and cook 1 minute.
Gradually add the milk, stirring until mixture thickens. Remove from heat and stir in the green beans, mushrooms, pecans, cheddar cheese, Worcestershire sauce, Tabasco, salt and pepper. Spread mixture in a 10" baking dish.
Preheat oven to 350°F.
To make Biscuit Topping, combine flour, sugar, baking powder, baking soda, and salt. Cut in the butter with a pastry blender or two knives until mixture is crumbly. Stir in buttermilk with a few quick strokes.
Turn dough out onto a well-floured surface and pat out to 1/2" thickness. Cut the dough into random-sized pieces and shapes and arrange on top of vegetable filling in a kind of crazy-quilt pattern, letting the filling poke through in spots.
Bake cobbler 25 minutes or until filling bubbles and biscuit topping is golden. Cool 5 minutes before serving.