In pot, bring stock to boil. Stir in rice and cook until tender, 8-10 min.
Beat yolks and lemon juice in large bowl. When rice is tender, temper yolks by adding hot broth to bowl one ladle at a time whisking constantly. After incorporating 1/2 the broth, add yolk mixture back to pan, whisking constantly, and cook over low heat until thickened but not boiling.