Slice the English cucumber into thin, even rounds and sprinkle with salt. Let sweat for 10 minutes, then pat dry.

In a mixing bowl, combine softened cream cheese and mayonnaise. Whisk until smooth, then gradually stir in soy sauce, rice vinegar, and sesame oil.

Toss cucumber slices with chopped onion and half of the dressing. Let sit for 5 minutes.

Gently fold in optional mix-ins and the remaining dressing until well combined.

Transfer to a serving platter and garnish with sesame seeds or nori. Serve chilled.