Note that cooking by HF waves was already familiar in industrial and medical applications. This was the first suggestion of home use for “bun sandwiches”. [Was hamburger trademarked? Apparently not, but cheeseburger was trademarked.]
Restaurants were starting to use HF ovens around 1960. I remember trying an electronic hamburger in a roadside cafe. The oven was fascinating, but the hamburger was too dry and tasted wrong.