He said 渋い or "shibui" to describe Nari's food. While shibui can refer to the bitter taste of a food, shibui is a word that can mean a lot of things. In this specific case shibui here refers to a dark/brown color. So he said something like "It's so brown/dark" it refers to how the food as a whole have a darker, heavier tone in color. Japanese mind the color of the food so much that it even taught in elementary school and became a basis in their school lunches. So a "healthy" meal is one that is "colorful". Brown/dark color is associated with heavy/serious meal. Plus, the food that Nari cooked would probably suit a colder season like winter or autumn (where the ingredients are more of that season's staple) while it's still summer in story. That's why he's wondering if it's the right season for this kind of food.