Cut tenderloin lengthwise to within 1/2' of bottom.
Place frozen lobster tails in boiling salted water to cover. Simmer for 5 minutes (tails should be slightly undercooked).
Remove lobster from shells as intact as possible. Cut in half.
Place lobster end to end inside beef.
Combine melted margarine and lemon juice. Drizzle on lobster.
Close meat around lobster and tie roast together with string or fishing line or whatever.
Place in roasting pan on rack. Roast at 425 for 25-30 min.
Place bacon on top; roast 5 minutes longer.
Sauté onion in margarine in saucepan, add wine and garlic salt and heat.
Slice roast, spoon on wine sauce and serve with a salad, vegetable, and hot rolls.