1-openModal.html

<template name="openModal">
  <button class="button" {{bind 'openModal: myModalTemplate'}}>Open modal</button>
</template>

1-openModal.js

ViewModel.addBinding("openModal", {
  on: "click",

  // Open Semantic UI modal
  get(event, elem, prop) {
    let template_instance = this.templateInstance,
        container = $(".ui-dimmer").get(0),
        parent_view = template_instance.parent().view;

    if (container)
      Blaze.renderWithData(Template[this.args[0]], this.data, container, parent_view);
  }
}, {
  // This binding doesn't need a viewmodel
  detached: true
});

2-modal.html

<template name="modal">
  <aside class="ui {{#if small}}small{{/if}} modal" {{bind 'modal'}}>
    {{> Template.contentBlock}}
  </aside>
</template>

2-modal.js

ViewModel.addBinding("modal", {
  // Initialize Semantic UI modal
  init($elem) {
    let options = {
      detachable: false,
      context: $elem.parent(),
      duration: 0,
      selector: {
        deny: ".actions .deny",
        approve: ".actions .approve"
      },

      onApprove() {
        // Modals should be closed explicitly after a successful operation
        return false;
      },
      
      onHidden() {
        // Get the outer modal view
        let parent_view = Blaze.getView(this).templateInstance().parent().view;

        // Destroy modal after hide transition
        Blaze.remove(parent_view);
      }
    };

    // Init modal on element
    $elem.modal(options);

    // Show modal immediately upon render
    $elem.modal("show");
  }
}, {
  // This binding doesn't need a viewmodel
  detached: true
});