Heat olive oil in a skillet over medium heat and sauté mushrooms until browned and moisture has evaporated, about 7–8 minutes.
Add garlic and spinach if using, cook until wilted, then season with salt, pepper, and thyme. Set aside.
In a saucepan, melt 2 tablespoons of butter, whisk in 2 tablespoons of flour and cook for 1–2 minutes to make a roux.
Slowly pour in milk, whisking constantly until thickened into béchamel sauce. Season with salt, pepper, and a pinch of nutmeg.
Lightly toast bread slices, then spread Dijon mustard on one side of each slice.
On half the slices, layer mushroom mixture, a spoonful of béchamel, and grated gruyère. Top with the remaining slices.
Butter the outsides of the sandwiches and grill in a skillet over medium heat until golden and crisp.
Place sandwiches on a baking sheet, top with béchamel and more cheese, and broil for 2–3 minutes until bubbly and browned.
Serve hot with a side salad or crispy greens.