Spread the outside of each English muffin half with about ½ teaspoon mayonnaise.
Pile ¼ cup of Cheddar and ¼ cup Monterey Jack on the bottom half of each bun, then top with the top half, mayo sides facing out.
Place the assembled sandwiches in a cast iron skillet or on a griddle set over medium-low heat (or on a grate over hot coals), and cook, flipping once, until the outsides are toasted and golden brown and the cheese is melted, about 5 minutes per side.
Slice in half and devour.
Notes
Skip the pre-shredded cheese--Pre-shredded cheese doesn't melt well. Skip the bagged stuff and grate it yourself from a block.
Let it rest--Let your sandwich sit for a minute before slicing it. They are quite hot immediately after cooking, and the cheese is runny. Letting them rest will allow everything to firm up and cool slightly.