Begin by roasting the chiles over the flame on your stovetop (or under the broiler) until the skin is totally blackened (if using fresh). When the chiles are totally charred, throw them into a plastic bag. Seal and allow them to steam and cool.
Next, pour the white vinegar over the sliced red onion and let them sit awhile. This'll take away a little of the crazy onion flavor and leave them with a nice sweetness.
Finally, make the special sauce: equal amounts of mayonnaise and Dijon. It's the simple things in life!
When you're ready to make the sandwiches, scrape the blackened skin off the chiles. Scrape out the seeds, too, so you'll wind up with big pieces of roasted green chiles.
For each sandwich, generously spread 2 pieces of rye bread with the special sauce. Lay on 2 slices of provolone, 2 slices of tomato, 2 slices of Cheddar... a nice layer of roasted green chiles... a few red onion slices... and one more slice of provolone just to hold it all together.
Top with the other slice of bread and spread the outsides with a good amount of butter.
Toast the sandwich in a skillet over medium-low heat until the cheese is thoroughly melted inside. Be careful not to burn the bread!