Preheat oven to 350° and lightly oil a casserole dish.
Mix together the cashews, lemon juice, 2 tablespoons of nutritional yeast, non-dairy milk, and 1/2 teaspoon in a high-speed blender or food processor until smooth and creamy. Set aside.
Heat the oil in a large pan over medium high heat. Add the onions and cook for 10-15 minutes, stirring often.
Add the garlic and kale to the pan, and stir until the kale is wilted. Add a little water if the pan is too dry. You may need to add the kale in two bunches, if your pan is on the small side.
Mix together the panko breadcrumbs with the remaining 2 tablespoons of nutritional yeast, dry mustard powder, pepper, and red pepper flakes. Set aside.
Remove the vegetables from the heat and mix in the cashew cream.
Pour the mixture into the casserole dish and top with the breadcrumb mixture.
Bake for 20 to 30 minutes, or until the top is golden and the casserole is bubbly.