I have this metal pan used with joint compound for taping drywall.
I fill it about 1/2 way up with water and fire up the grill on high.
Let the water boil for about 10 minutes, so that steam is coming out
all around the grill.
Shut it down, tip over the water with a long handled something (I
usually do this through the rotisserie hole with the lid closed.
Wait about 5 minutes then hose the whole thing down with hard spray
nozzle on the garden hose.
It usually comes cleaner than a whistle!
The few particles that remain can be wiped off with a wet rag.