Philadelphia Roll

Name: Philadelphia Roll
Ingredients: 1/2 small red onion, thinly sliced; 1/2 c. unseasoned rice vinegar; 2/3 c. sushi rice; 2/3 c. water, plus more for rinsing rice; 1 (2) piece kombu (optional); 1 tbsp. unseasoned rice vinegar; 2 tsp. granulated sugar; 1/4 tsp. kosher salt; 2 sheets nori; 2 tbsp. everything bagel seasoning, divided; 4 oz. smoked salmon, divided; 4 oz. cold cream cheese, cut into 1/2-thick strips, divided; 1 Persian cucumber, seeds removed and julienned, divided; Soy sauce, wasabi, and pickled ginger, for serving