Preheat oven to 450F

Heat a medium pot of salted water on high

Thinly slice mushrooms and kale (remove and discard the kale stems)

Peel sweet potato, thinly slice into rounds

Zest lemon to get 2 tsp and quarter lemon

Roughly chop pistachios

Peel and mince garlic

Core and thinly slice apple

Heat 2 tbsp oil on medium-high heat, add mushrooms and cook for 4-6 minutes

Add kale and cook until wilted

Add ¼ cup of water, cook until water has cooked off

Turn off heat and stir in lemon zest, season with salt and pepper. Set aside

While the vegetables cook, add sweet potatoes to boiling water and cook for 12-14 minutes

For the sauce: heat butter until melted, add garlic, add flour and cook, whisking frequently until thoroughly combined. Slowly whisk in milk and ½ cup of water until thickened. Remove from heat and season with salt and pepper

Spread ⅓ cup sauce on bottom of baking dish. Evenly top with ⅓ sweet potaotes then ⅓ cooked vegetables and repeat twice to make layers. Evenly top with parmesan cheese. Bake 14 to 16 minutes or until golden brown. Let cool

While the gratin is baking make salad with lettuce, apple, blue cheese, pistachios and dressing of lemon, salt, pepper and olive oil.