Combine ¼ cup cottage cheese, 1 teaspoon miso, 1 teaspoon soy sauce and ½ teaspoon rice vinegar in a mini food processor; process until smooth, about 1 minute, stopping to scrape down sides as needed. Transfer to a small bowl; stir in 1 tablespoon scallion and ½ teaspoon sesame seeds. Spread half of the cottage cheese mixture on each slice of toast.

Toss ⅓ cup cucumber, the carrot ribbons and 2 tablespoons radishes with the remaining 1 teaspoon rice vinegar in a medium bowl until evenly coated. Layer 1 toast slice with vegetable mixture and sliced avocado; top with the remaining slice, spread-side down. Cut the sandwich in half to serve.