Well in this case, just simply go to the Theme Editor -> Sections -> Menu then create a Mega Menu

1. Enable Megamenu -> Enable Megamenu as Collection Grid Only (once enabling the option, it would only display Collection Grid regardless of the options you have attached in that particular Megamenu)
2. Just simply attach a Menu with the list of collections you want to display with: http://prntscr.com/rbtzda (and you're good!)