each in

controllers.application.js

import Ember from 'ember';

class MyObject {
  constructor(name) {
    this.name = name; 
  }
}

export default Ember.Controller.extend({
  items: {
    'A': [new MyObject('one')],
    'B': [new MyObject('two')],
    'C': [new MyObject('three')],
    '1.D': [new MyObject('four')]
  }
});

templates.application.hbs

{{#each-in this.items as |key value|}}
	{{#if value.length}}
    {{key}}<br>
    {{#each value as |item|}}
      &nbsp;&nbsp; {{item.name}}<br>
    {{/each}}
  {{/if}}
{{/each-in}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}