<?xml version="1.0"?>
<!--
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <type name="MagentoCatalogModelResourceModelProductCollection">
        <plugin name="sidebarFilters" type="SebwiteSidebarModelPluginSidebarFilter"/>
    </type>
</config>

<?php namespace SebwiteSidebarModelPlugin;

use MagentoCatalogModelResourceModelProductCollection as ProductCollection;

class SidebarFilter
{
    /**
     * aroundAddFieldToFilter method
     *
     * @param MagentoCatalogModelResourceModelProductCollection $collection
     * @param Closure                                                $proceed
     * @param                                                         $fields
     * @param null                                                    $condition
     *
     * @return MagentoCatalogModelResourceModelProductCollection
     */
    public function aroundAddFieldToFilter(ProductCollection $collection, Closure $proceed, $fields, $condition = null)
    {
        // Here we can modify the collection

        return $fields ? $proceed($fields, $condition) : $collection;
    }
}