index.erb

<form action="/" method="POST">
  <h3> Messaggio </h3>
  <input type="text" name="message" value=<%= params[:message] %>>
  <h3> Email </h3>
  <input type="text" name="email" value=<%= params[:email] %>>
  <input type="submit" value="Inserisci ">
</form>


<% @messages.each do |m| %>
  <p><%= m[:email] %> </p> 
  <p><%= m[:message] %> </p>
<% end %>

main.rb

require 'sinatra'
require 'pry'


class Form
  attr_reader :messages
  def initialize
    @messages = []
  end
end

f = Form.new

get "/" do
  @messages = f.messages
  erb :index
end

post "/" do
  @messages = f.messages
  @messages << {email:params[:email] , message: params[:message]}
  erb :index
end