Melt butter in a large nonstick skillet over medium-high heat. Add shallots, onion, and garlic to pan; sauté 3 minutes. Add mushrooms, oregano, thyme, and 1/2 teaspoon salt to pan; sauté 5 minutes or until liquid evaporates and mushrooms darken. Add sherry to pan; cook 2 minutes or until liquid evaporates. Transfer mixture to a large bowl. Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1 cup. Stir breadcrumbs into mushroom mixture. Cool.
Slice beef lengthwise, cutting to, but not through, other side. Open the halves, laying beef flat. Slice each half lengthwise, cutting to, but not through, other side. Place heavy-duty plastic wrap over beef; pound to an even thickness using a meat mallet or small heavy skillet. Spread mushroom mixture down center of beef, leaving a 1/2-inch border. Roll up beef, jelly-roll fashion, starting with long side. Secure at 2-inch intervals with heavy string. Rub beef with remaining 1 teaspoon salt and pepper.
Place beef on a broiler pan coated with cooking spray. Bake at 450° for 40 minutes or until a thermometer registers 135° or until desired degree of doneness. Remove beef from oven; let stand 10 minutes. Slice.