Cut tenderloin lengthwise to within 1/2" of the bottom. Place frozen lobster tails in boiling water to cover; simmer for 5 minutes. Remove lobster from shells. Cut in half. Place lobster end to end inside the beef. Combine melted butter and lemon juice. Drizzle on lobster. Close meat around the lobster and tie roast together with string. Place in a roasting pan on a rack. Roast at 425~ for 30 minutes. Place bacon on top, roast 5 minutes longer. Saute onion in 1/2 cup butter in saucepan, add wine and garlic salt. Heat through. Slice roast. Spoon on the wine sauce. Serve. NOTE: The tenderloin can be stuffed and tied ahead of time and then refrigerated until ready to cook.