Kelly Clarkson -- Stuffed Mushrooms ... Key To Singing LIVE

Wanna know the secret to Kelly Clarkson's sensational LIVE performance of "My Country, 'Tis of Thee" at the Presidential Inauguration? Two words ... STUFFED MUSHROOMS.

That's right, the night before Kelly rocked the National Mall in front of more than 800,000 people ... she and a few friends carbo-loaded at Buca di Beppo in D.C. -- and as everyone knows by now, it clearly paid off.

We're told Kelly and co. feasted on lasagna, bruschetta, stuffed mushrooms, half-pound meatballs, spaghetti, chicken parmigiana -- and the always delicious colossal brownie sundae ...all for a cool $230.

Our sources say Kelly was super friendly, took time to snap pics with the staff (above) -- and left in extremely good spirits.

And then she did this ...