Toast bread and spread with uni butter. Top toast with remaining ingredients and serve.
For uni butter, process 1 pound soft butter in a food processor until smooth, adding 100 grams uni, piece by piece; shape into a log with plastic wrap. For mushrooms, sauté 1 pound baby shiitake mushrooms in 1 tablespoon canola oil until dark golden brown, season and let cool. For sake-compressed pear, cut 1 Asian pear into a 1/2-inch dice and combine with 1/2 tablespoon sake and 1/2 tablespoon yuzu juice in a small vacuum bag; seal on high and let marinate for at least 4 hours.