Kastner, born in the Czech Republic, trained as a blacksmith and spent some time restoring historical metalworks at a castle in Western Bohemia before moving on to natural materials design and sculpture. He founded Crucial Detail in 1998, shortly after his arrival in the US. He is best known for his Alinea serviceware concepts, which landed him on The Future Laboratory's list of 100 most influential individuals in contemporary design.

He is also the designer behind The Porthole Infuser, a beautiful infusion vessel originally created as one of many custom pieces for bar chefs at The Aviary in Chicago. The Porthole has been featured in dozens of publications, including the cover of Food Arts Magazine, eventually finding its way into the world's most prestigious bars, hotels, and restaurants as well as the Museum of Modern Art store in New York City. Kastner received the 2014 IHHA Global Innovation Award for Best Product Design and is the designer behind Team USA platter and tools for Bocuse d’Or 2015 and 2017, helping Team USA win silver and gold for the first time in history.