Neither of these things are guaranteed to happen if you get the flu vaccine when you are sick, but they are possibilities. If you have just a mild illness, there is no reason to wait to get vaccinated.
Early in the day, let roast stand at room temperature at least 1 hour before cooking. Preheat oven to 375 degrees. Rub meat ... the outside and rare in the middle. Meat will be very brown ... stand at room temperature for at least one hour.
Preheat oven to 500 degrees. Wait for a few minutes before you put your roast in, so it gets as hot as possible. Put the roast in a shallow pan. (I use a cookie sheet). Cut onion into thin rings, and put all over the top and sides of the roast using toothpicks if necessary.