as_modal.js.haml

:plain
  var existingModal = document.querySelector("[data-controller='modal']");
  if (existingModal) {
    document.body.removeChild(existingModal);
  }

  document.body.insertAdjacentHTML("beforeend", "#{j render partial: template.to_s}");

controller.rb

class CommercialPayersController < ApplicationController
  def edit
    render :as_modal, locals: { template: "form" }
  end

  def update
    if @payer.update(payer_params)
      redirect_to @payer, notice: "Successfully updated Commercial Payer"
    else
      render :as_modal, locals: { template: "form" }
    end
  end
end

form.haml

// note you could move the "modal" markup to shared template if you want
.modal(data-controller="modal")
  .modal-content.w-600
    = form_with model: @payer do |f|
      %h2.font-light.text-3xl.text-grey-700.mb-8
        Edit Payer

      .input-group
        = f.label :name, "Payer Name"
        = f.text_field :name, placeholder: "Enter a name..."
        
      .input-group
        = f.label :description
        = f.text_field :description, placeholder: "Enter a short description..."

      .text-right
        .btn-secondary.mr-4(data-action="click->modal#close")
          Cancel
        = f.submit "Save", class: "btn"