This style of pasta is very popular in Japan. You can find packaged ume/shiso flavorings for pasta, but nothing beats the homemade version. The first part of this recipe consists of making sweetened ume paste called ume bishio. It is something most Japanese don't even know how to make anymore and it is a shame. It is super versatile and can be used for many recipes. It keeps forever in refrigerator too. For this recipe, you only need to use 3 T of ume bishio and you can keep the rest in the refrigerator for later use. This sauce will only barely coat the spaghetti and is not meant as a thick sauce such as marinara and more like Japanese style pesto sauce.