Cook pasta as directed on packet. Drain.
In a saucepan, melt butter and mix in flour. Cook over a low heat for 1-2 minutes until a smooth paste. Pour in milk and cook, stirring, for 5-10 minutes until sauce becomes thick.
Add egg and quickly mix. Add tuna and tomatoes and stir to combine.
Combine pasta and sauce in pan, stir then put lid on pan for 5 minutes to allow excess sauce to absorb into pasta.