What can i feed my 11 month old english mastiff to gain weight?

You don't want her to gain weight too fast as it will have a negative impact on her hips and elbows. Has the vet said she is too thin? Have you talked to the breeder? Mastiffs aren't fuly filled out until 18 months- 2 years old. If you feel she is still too thin at that time, then seek a vet's advice.