Heat a large skillet over medium heat and spray with cooking spray. Add mushrooms and cook 3 – 5 minutes until tender. Add kale and thyme and cook another 2 – 3 minutes to lightly wilt kale. Season with salt and pepper to taste.
Spread 1 teaspoon Dijon mustard over one slice of bread. Layer ham and Swiss cheese, then half the mushroom and kale mixture. This will pile on, but that’s what you want.
Top sandwich with another slice of bread and lightly spray with olive oil cooking spray. Place the slide with cooking spray down on a panini grill, then lightly spray the other side of the bread with cooking spray.