Cut tenderloin lengthwise to within 1/2” of bottom to butterfly.
Place lobster tails in boiling salted water just to cover. Return to boiling; reduce heat and simmer lobster for 5 to 6 minutes.
Carefully remove lobster from shells. Cut lobsters in half lengthwise. Place lobster, end to end, inside beef tenderloin.
Combine 1 tablespoon melted butter and lemon juice. Drizzle on lobster.
Close beef tenderloin around lobster; tie roast together securely with string at intervals of 1”.
Place stuffed beef tenderloin on rack in shallow roasting pan.
Roast the stuffed tenderloin for 20 minutes for rare doneness.
Lay bacon slices atop of the beef tenderloin; roast the tenderloin for 5 minutes more.
Meanwhile, in saucepan, cook scallions in remaining butter over very low heat until tender, stirring frequently. Add wine and crushed garlic and heat through, stirring frequently.
To serve Lobster Stuffed Tenderloin, slice roast, spoon on wine sauce. Garnish platter with whole mushrooms and watercress.