Puree tomatoes in a blender until smooth, adding a tablespoon or 2 of water if dry. Reserve.
Fill a large stockpot with water and bring to a rolling boil. Add salt and pasta and cook until al dente, about 8 minutes. Drain in a colander.
Melt 4 tablespoons of butter in a large skillet over high heat. Saute shrimp with salt, pepper and red pepper flakes until pinkorange, about 1 minute per side. Add tequila and flambe. With a slotted spoon, transfer shrimp to a platter, leaving butter in pan.
Add reserved tomato puree, bring to a boil and cook until reduced by about one third. Adjust seasonings to taste.
Break apart remaining butter into small pieces and stir into sauce along with shrimp. Once smooth, remove from heat, and ladle over bowls of warm fettuccine. Garnish with avocado and chopped cilantro. Serve immediately.
c.1997, M.S. Milliken & S. Feniger, all rights reserved