In a small bowl, mix together the mayonnaise and the Dijon mustard.
In another small bowl, mix together the grated Wisconsin cheddar and Wisconsin gouda cheeses.
Heat a large skillet over medium-low heat.
Spread the mayonnaise and Dijon mixture on the inside of each slice of bread
On one slice that is spread with the mayonnaise mixture, place about 1/3 cup of the cheese.
Place the arugula / spinach mix on top of the cheese.
Add the roast beef.
Top the roast beef with the caramelized onions and plum chutney.
Add the tomatoes.
Cover with the remaining cheese.
Top with the slice of Rye bread.
Melt one tablespoon of butter in the pan, focusing on the area where the sandwich will be.
Gently place the entire sandwich into the pan.
Cook for approximately 7 minutes, checking the bottom with a thin spatula after 5 minutes.
When the bread is golden brown, lift the sandwich up with the spatula and add another tablespoon of butter (or more, of necessary) to the pan and allow it to melt.
Gently flip the sandwich over and return to the pan to cook for an additional 7 minutes, again checking the bottom at the 5 minute mark.
Do not be tempted to turn up the heat. We want it to cook low and slow and allow the cheese to melt perfectly without burning the bread.
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