Installing American countertop oven in Europe - Converter Trouble
Yes.  First, that was a very foolish thing to do... though I'd make an exception for certain built-in ovens whose guts are mainly 240V but have a small amount of 120V conntrols.  

The simple fact is: The machinery which turns electricity into heat is very simple, which means heaters are inexpensive. 

And one more thing: American heaters are crippled. You may notice it is 1500 watts, not a surprise. Every US heating appliance is 1500W - toaster ovens, hair dryers, curlers, heater-fans, kettles, microwaves, plug-in wall heaters, you name it. If it has a NEMA 5-15 plug, it's 1500W because that is the practical capacity of a 15A 120V circuit.  

In Europe they don't have that problem.  Their sockets support up to 2x what ours do, and so their heat-based appliances are bigger when that would make a better appliance.  If you really wanted to ship appliances across oceans, you'd be fitting Europlugs or BS 1363's in your US kitchen.  (which is possible, by the way).

So.  Simply buy a European version of what you want. Done.  As for the 120V unit, stick it on Euro Craigslist.  Some expat may be permanently returning to the US and be willing to throw it in their container.  Given how inherently inexpensive heating appliances are, I wouldn't ship it in a box. 

OK, so you Really, Really Want that 120V appliance

On UK construction sites, they use 110V for big contractor-grade power tools.  They use a special transformer which gives them 110v "center grounded" so each hot is only 55 volts from earth.  That makes the tools much safer than 240V tools.  US heat-based appliances will cheerfully run on that, as the transformers are plenty big.  So get one of those transformers and you're all set. And it has resale value. 

One last thing to try

If the heater is very bespoke specialty, i.e. actually built by hand in a western country, then the manufacturer may have a phone number they answer.  Ask them if they market this unit internationally.  Ask them if there's a procedure to convert it from 120V to 240V.  Worst thing they'll say is "no", best thing they can say is "Rearrange some jumpers".