This is an example of a server side rendered block without any build step. It's all plain JS/PHP and it uses the helpers and components WordPress already offers in the block admin interface.

block-ssr.js

(function (blocks, components, element) {
	blocks.registerBlockType('test/ssr', {
		title: 'Server Side Rendered Block',
		icon: 'admin-site-alt3',
		category: 'test',
		edit: function (props) {
			return element.createElement(components.ServerSideRender, {block: 'test/ssr'});
		},
		save: function () {
			return null;
		},
	});
})(wp.blocks, wp.components, wp.element);

block-ssr.php

<?php
add_action('enqueue_block_editor_assets', function () {
    wp_enqueue_script('block-ssr-script', '/SRC_TO_JS/block-ssr.js', ['wp-blocks', 'wp-element', 'wp-edit-post', 'lodash'], '1.0.0');
});

add_filter('block_categories', function ($categories) {
    return array_merge($categories, [
        [
	    'slug'  => 'test',
	    'title' => 'Test Blocks',
	],
    ]);
});

register_block_type('test/ssr', [
    'render_callback' => function () {
	return 'Hello! I am rendered on the server.';
    },
]);