Classic comfort food.
This is one of America's favorite dishes; but are they separate (two things), or are they a single thing…one dish?
So…should you say, "Mashed potatoes and gravy ARE my favorite," or "Mashed potatoes and gravy IS my favorite."?
If you consider that mashed potatoes & gravy is a single dish — not two separate, distinct items — you must vote for "is".
And you would be right! "Ham & eggs"…another matter for another day.