Two, no three ways I can think of...
First way, easiest and most expensive while having an industrial touch and just enough showbiz bling:
Circuit Specialists 16VA520T20 Variac Variable Transformer, 20 Amp, 2000VA Max, 0-130V Output
It will also burn out the first time you turn it on with the wiper anywhere except endstop. But maybe not. Devils in the details.
Next, a cheap motor speed controller available at the local dope plant hydroponics store might work, like this one:
https://www.amazon.com/MLCS-9410-20-Amp ... 001NIK6PC/
They're made for dirty switching inductive loads like electric motors, but the simple resistive heater element might be okay.
--> these things are $50 on the shelf at the right store and I suspect it will work *IF* and ONLY IF the amperage rating is a *FULL* 20A. No lying to yourself, this is a very easy way to turn cash into blue smoke.
Most esoteric, persnicketiest, *and* riskiest on the budget solution: Modulate the power to the element using a microcontroller produced control pulse into a solid state relay...
Digikey (digikey.com) has zero-crossing solid state relays, devices that handle 1500 watts (!) are going to cost some money. A solid state relay is like a regular relay but doesn't actually have mechanical contacts, just uses transistors inside. They can handle AC and/or DC per ratings, and the amperage rating is generally tied to the cost, 20 amps is gonna be a little pricey. Zero-crossing SSRs can change state in "Max. 1/2 cycle of voltage sine wave + 1ms", or in other words, plenty of time for you to switch a heater on and off multiple times a second. This ( http://www.digikey.com/product-detail/e ... -ND/307696
) is a solid state relay that looks like it could handle that element. It's the cheapest of the list, so they start around $30 USD and go up.
That's a little depressing, as it will also need heat sinking (it is inherently an inefficient device) which may also include forced air cooling with a small fan. An old style electromechanical magnetic coil relay *can* be used, but the contacts and ratings of the relay will specify a lifetime, you're only gonna get a half million clicks or so, and it's gonna do this as fast as it can, like CLICK CLICK CLICK CLICK all the time and that is gonna GET on your NERVES, it makes the cost of the SSR seem reasonable really fast.
So using a variable dwell PWM signal with a minimum on/off time, we can adjust for the type of relay used. You don't want to sit there touching the stupid wires together yourself, so we need a small microcontroller to do that, and that can be just about anything from an Arduino Atmel chip to a five dollar Raspberry Pi Zero.
So... Big honking relay thing bolted to something cold, controller, and twistknob button something you can use to adjust the power up and down. If you use the Raspberry Pi option, you can have a bloody iPhone app that lets you adjust your steamer temp with it, but hey, I'm not writing that.
Now here's the funny part...
I just designed a circuit more or less to do this exact task. It features a Cypress PSoC uController driving a 20A relay with a 5V USB power source, a thumbnail sized OLED display, four silly tactile switches for program config, and a thermocouple input. I got 32K of flash EPROM that will hold about 6 pages of 'C' code which isn't written yet. I got PC boards for it last Friday DHL Hong Kong. I'll post pics when I cook it up, maybe today. I was gonna use it to control an oven for PC board manufacture