The first thing you need to know about the “Hemings & Hercules” dinner at Post and Beam in Los Angeles is that it’s an eight-course tour de force of open-air, wood-fire, cast-iron cookery dedicated to two of America’s first celebrity chefs—James Hemings and Hercules Posey. The second thing you need to know is that both men were African American slaves owned by two of our Founding Fathers—George Washington and Thomas Jefferson. I first became aware of this little-known aspect of America’s culinary history when I watched the third episode (“Our Founding Chefs”) of the Netflix series “High on the Hog: How African American Cuisine Transformed America.” It introduced a pair of Los Angeles chefs (Brian Dunsmoor and Martin Draluck) who were dedicated to providing an intimate dining experience that could combine the story of “Hemings and Hercules” with a succession of sumptuous dishes prepared in the blazing-hot, open-hearth “hard-cooking” tradition …