What to say to boyfriend's mother who you are meeting for the first time

I would compliment her on something. It has to be a heart felt compliment. Then ask her how you can do the same thing at home. If you love her roast, ask for the recipe and some tips on making it. You want to create the scenario of mother knows best. That you admire her and how she keeps her house and cooks her food. But, remember, above all, be yourself.