Instead of broccoli, replace with zucchini, or other low FODMAP vegetable.
Low FODMAP gravy mix powder: Gravox in Australia has no onion or garlic, but if you cannot find suitable gravy powder, use 3 tablespoons corn flour as a thickener and 1 tablespoon tamari sauce.
Massel 7's Bouillon Cubes contain no garlic nor onion and can be used to make the broth.
For the mashed potato: There are hundreds types of potatoes around the world, but most chefs agree that the best potatoes for mashing are the starchy types.
Depending in which country you live in, there are different types that are suitable.
Just to give you an idea in Australia go for Coliban, Dutch Creams, Golden delights, Sebago.
In the USA select Russets, or all-purpose potatoes like the Yukon golds.
In the UK Maris Piper potatoes seem to be Jamie Oliver's favourite for mashing, another one is Desiree.