Melt a good sized pat of butter over low to medium heat
Toss in garlic and stir - about 30 seconds just to warm it up, don't cook it before adding the mushrooms.
Add mushrooms and sauté, adding butter if needed and stirring occasionally
As mushrooms brown and shrink in a second saute pan on medium low heat or on the other end of the griddle add spinach until wilted.
Once spinach is wilted and mushrooms are done to your liking, remove vegetables from heat to separate bowls, or separate ends of a cutting board.
Butter the pan/griddle, and arrange bread over the heat, add cheese to each slice of bread (you should have 2 slices with cheddar, and 2 with swiss) and keep an eye on them as they warm up and the cheese starts to melt. As they go, move them around so the bread browns evenly.
As the cheese starts to melt, once it's sticking nicely to the bread, it's time to stack the sandwiches, I make the least toasted slice the bottom, then, I'm picky about order - so, let's say that's the edam/swiss, add the mushrooms, then the spinach, then turn the cheddar slice of bread over to top the sandwhich. Finish grilling the sandwhich as a whole, sometimes I flip it once or twice, sometimes, it's pretty much done when assembled.